I built a battery box out of a Project Box from Amazon. I added a vent on the box, which is recommended if you're gonna recharge it, and an in-line fuse.
I put a 1" hole in the box to bulkhead mount a SAE connector, and put the matching connector on both my fish finder and the battery charger.
I went a little overboard (ME? Never!) and put about $60 (plus the battery
) into it, but it works. I got some velcro at Malmart that I gooped down to the inside of the hull to keep it from sliding around. It's good enough for paddling, but I have to take it out when hauling it in the truck.
For a charger, I bought the matched charger at Gander Mountain when I bought the battery. I rigged up a spare SAE connector to attach the charger to my battery via the alligator clips that came with the charger.
A little advice, if you go that route, make one of the leads shorter than the other, or insulate it, so you don't blow your fuse. Experience points: 2.
If anyone is interested, I will take a few pics to add to the post later.