I keep mine inside the yak in a block of styrofoam that I try to wedge into the bow. It's better than my solution last year, but I'm still unhappy with it. The battery shifts in transporting the kayak or when the water is rough, sometimes causing terminals to come loose. The battery also doesn't stay completely dry (it gets condensation) which I worry will damage it over time.
If anyone has bought or made something that they are happy with, please share!