Very Nice!!

You can use this and mount a boat seat to your cooler.
Boat Seat Bracket

Won this seat a couple of years ago at a kayak builders meet.

RBR, that is absolutely beautiful!! I have been thinking about a SUP for a couple of years, but my weight has been a barrier. But to build one, I could build it as big as I need. How big do you think it would need to be to support my 265# plus a little tackle? For exactly the same use as you want it for.

There is a large variety of SUPs available. You are not nearly so heavy that you could not find one that would support you.
The big advantage to building one is that it can be lighter than anything you can buy although not as tough.
So if you intend to build one, I will make a few suggestion.
If you plan to fish shallows the catamaran style hull eliminates the need for skeg. A skeg would be a big annoyance in that case. And the catamaran style hull eliminates the need for a handle.
You want floatation equal to about twice your loaded weight. Water weighs 64 lbs/cubic foot. So you are going to need 8 to 10 cubic feet of foam. Owens Corning (pink stuff) is the better choice for an SUP. It is available up to 4 inches thick.
I like the 12 foot length because it does not require a flag to transport in my truck. You may want longer.
I like the 32" width because it is easy to paddle. The wider it is the more wiggle waggle. But this about the minimum for stand up stability. Youtube videos abound as to how to mount etc.
Mine is glassed all over and then with layer of Kevlar/Graphite where the load is. This does 2 things, adds toughness and foot grip. But Kevlar is nasty stuff. Will require special attention to make those bacony edges lay down. Dremel works OK. Dynafile works best.
Mine has rocker but you don't need rocker as long as the nose comes up a bit.
You may note how I attached hardware. A screw into foam is not going to hold.
You definitely want white colorant for the top. It is a lot cooler.
Gorilla glue or epoxy are your best glues.
Did I miss anything?

That's great information! I'm seriously thinking about trying my hand at building one this winter. I will probably make it about 14' long just to maximize the carrying capacity and improve speed. Quick question-why is the pink foam board better than, say, the blue foam board that are commonly available?

Pink is slightly lighter. Either will work. Pink is no less available.
Get the blue at Lowes. Get the pink and Home Depot.
Probably shouldn't be less than 32" wide.

Just doing some basic math, if I build it 32" wide, 14' long and 2" thick, and build the pontoons 8" wide and 3" thick, I should have a capacity of about 500# with just a little freeboard. Would it be strong enough? Are you glassing wood to the surface for mounting points? I would roll the front upwards a couple of inches, but I don't want rocker. I believe the hull would be faster without it.

Some good ideas. But you may want to consider that foam board comes is 1/2", 3/4", 1", 2" and 4" thicknesses. You want to minimize the layering and scrap.
The pink stuff is 1.5 lbs per cubic foot. Blue stuff about 2 lbs per cubic foot. Light but not weightless. Glue is much heavier. Glass heavier still.
Your floatation should be about twice your capacity. Flotation can be calculated 64 lbs per cubic foot of foam.
The foam is probably strong enough all by itself by glassing it will make it much stiffer, stronger and more durable. I also ran a 1/4" by 3/4" wood stringer down the bottom corner of the pontoon for durability. Fiberglass is strong but brittle and you will want to drag your board up onto shore.
The catamaran configuration precludes the necessity of a handle and a skeg.
The wood mounting points are bonded in place and then a layer of epoxy for waterproofing. If you wanted a really slick surface you would imbed the wood. But even glassed foam alone won't hold a screw. You need the wood.
Also I used a piece of hybrid Kevlar/Graphite on the standing area for toughness and grip. You need 2 layers of fabric in the standing area and you need something to grip your feet.
Mine has rocker but I think that is unnecessary. The rocker wasn't for speed is was for turning ability and mine turns a little too good. In the wind it is difficult to drive straight.
1 inch upturn at the front should be sufficient with the bottom tapering upward.
May want to review my build.

http://www.texaskayakfisherman.com/forum/viewtopic.php?f=71&t=244566

I had read your build, but I just read it again. What did you make the wood stringers from, specifically? Why didn't you glue them directly to the foam? I think I may add some hardwood strips on the outside edge of the pontoons to add durability. I have some Ipe and Cumaru that I could use for that, as well as the stringers. Not the lightest, but very tough.

I'm thinking about wood embeds flush with the deck, and maybe even gear track. I may even make a groove for the gear track. Any thoughts about that?

Western Red cedar stringers are glued on top (bottom of the SUP actually) of the fiberglass.
For the mount points if I had know what I wanted before hand I might have embedded them in the foam and glassed over.
Wood stringers for toughness are a good idea. You will want your stringers on top of the glass but coated with epoxy for toughness and waterproofing. You want to watch your weight though. One of the big benefits of a DIY SUP is the light weight, and one hand carry. And you want the surface you glass as smooth as possible. Glass does not negotiate sharp corners well. Composite fabrics are even worse.
Gear tracks would be overkill IMO. But if you decide on gear track buy an aluminum track and mount to wood. A DIY track won't stand up to the abuse.

I can sort of "attest" to RBR's math, that the dimensions he has built for himself and is recommending for someone around 265 lbs. with some gear . . . seem spot on to me.

Below, a pic or two of me (255 lbs. or so here) a few weeks ago in my S-15, so 15'3" length and 38" width effectively (since I am nowhere near drafting enough water to get any benefit from the sponsons (boat collars)). I was just returning from a mile or so out standing and paddling all the way, then standing and paddling back to my boathouse. 2 miles, good speed, great balance. 15'3" is superfluous as I can carry along another big friend with no issues.

I really like the idea of homemade SUPs as RBR has shared here. These could be super fishing platforms in coves and various shallow areas.

Anyone who thinks fishing while seated the majority of the time in a SOT is not disadvantaged, well, I think they could be right for certain presentations but there are so many that benefit from being on your feet. Sighting fish, too, of course.

Oh! And the seat options RBR can employ? Many are cheap and more comfortable than the kayak options.

Brad

I would probably buy aluminum track from Rockler. A 24" section on each side would be plenty. If it is inset in the surface like it is on my Commander, then it won't snag anything. My only concern is the exact placement. I want to build some fore and aft adjustability in my seat mount, and the track placement will need to accomodate that.

Is there any reason not to use something harder for the stringers? I have some hardwoods available.

An inset track is not going to be easy to accomplish. I can think of a couple of ways to do it but nothing neat.
If you look at my stringer you can see that it is in position to take a lot of abuse. So harder can't hurt.

I will probably use the router to cut out a slot for wood backing, and then screw the track to the wood after glassing. Should I put glass under the wood before I glass over it? Or just epoxy the wood to the foam and then glass over it?

In fact, I plan to have everything laid out before I build it. Seat mounts, tie offs, cleats, etc. I want to place all of the wood backing flush with the surface, if possible.

I mostly use Gorilla glue when bonding foam to foam or wood to foam. Never have to worry about mixing quantities and it fills and sands nicely. It would be difficult to get glass in the bottom of a routed slot. Glass does not do corners well. So I would suggest just glass over. May want to run a 2" or 3" strip of fiberglass tape over the wood and then your cloth over that. You would need to sand those bacony edges of the tape so that the cloth will lay flat.
I found that a belt sander is a necessity, but a Dynafile is certainly a well used tool also. The Dynafile is the the "go to" tool for working the rough edges of anything Kevlar and glass too. And just about any kayak build requires a lot of sanding. I mean like it will be the majority of the project time spent.