My suggestion is just get a little bit bigger 12 volt battery with more amp hours. Makes much more since when it comes to having fewer wires/parts and being cheaper. Batteries plus in San Diego is where I got mine. They are reasonable priced and pretty nice. The best way to go would be to buy one of the battery boxes that comes pre-wired and with a battery box from the online store attached to this website.
http://www.kayakfishingsupplies.com/...e-Power/Detail