I originally built this box about 2 years ago, and did so without a whole lot of research, it worked but didn't have the 'fit and finish' I wanted, so I learned a bit and came up with this current version. I hope it helps anyone who needs to do this for their kayak.
I have a 7.2 amp/hr 12v SLA battery inside the box. I can also store my fish finder (up to elite 5 model) along with a pair of orange float/markers for marking positions and all wiring.
there is also a nice outer storage for tools, spare fuses, bolts, and any other item necessary related to the battery/fish finder.
Inside wiring is an sae connecter coming off the terminals, through a waterproof switch and out through the box through a cable gland (water tight seal). the connections to the switch have been soldered for corrosion resistance. all connections have used shrink material where possible.
then outside the box I have a sae splitter so I can run power to 2 devices (lights and fish finder) and the wiring for the fish finder is kept nicely inside a plastic shroud tube.
all can be disassembled and put inside the box for transportation.