The "blue brick" has been popular for a while; but it's only 9800maH.
14V 16000mAh LiPO4 batteries are coming down in price

16aH for under $80 in a two pound package was too good for me to pass up

They are traditionally used to power big drones

You will need a balance charger (these batteries typically don't have their own battery maintenance system)
The most popular balance charger is the IMAX B6 mini

Depending on the LiPO4 battery, I'd recommend HXT to T-terminal adapters to make charging the battery and powering the fish finder easier

I'd recommend using a fire proof battery bag (or you can DIY with a welding blanket)

These batteries are long and skinny; so the battery box you make would have those dimensions

Blue Brick

Example of 16mAH LiPO4 battery

Example of HXT to T-terminal adapter. There are different HXT sizes, so choose the specific size for the battery

Another option would be to wire two blue brick batteries in parallel; which may be a better option for some

Bioenno Power makes a true deep cycle lithium. I would caution using lithiums in parallel. There's a higher chance of damage to one of the batteries that way, which could be catastrophic. RC style lithium battery packs like that are meant to have a much higher discharge rate than what we could pull off of them in a kayak. Contact Anglers Pro Tackle in North Richland Hills I have been working with them to get set up as a dealer. I spoke with Bioenno about my current set up which is a 55 ah AGM sealed battery that weighs around 40 lbs. It will only give me around 6-8 hours before I start getting low voltage warnings from my graph. Bioenno advised I could use there 30ah battery and get more hours use than my current set up and the battery is much smaller weighing only 10 lbs.

Thanks for the info

You are right about wiring lithium batteries in parallel and the drone/rc lithium batteries� I use one to power an LED light bar on my kayak

The blue brick is now more widely available (Walmart now sells them online for $22)
I�ll post how I made the battery box for the blue brick when I get some time

I picked up one of these on clearance last year at walmart, i think normal price is 40 bucks.

https://www.walmart.com/ip/Stealth-Cam-12V-Battery-Box/6225663

I just cut the female power end off and soldered on a generic water proof trailer plug on it and the matching end to my Garmin power cable. This thing lasts me all day, it comes in a waterproof case with a handle, small enough form factor for me and with the soldering mods i made connects in 10 seconds. has last over a year already and holds a charge on an 8 hour fishing trip. I only use the garmin Striker 4DV so the draw isnt to much.