I use this product:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
See this post, and find the post by user Craig:
http://www.northwestkayakanglers.com...?topic=10523.0
Looks like this:
I got the 15Ah one (doesn't look like they sell them anymore), and have used it for 4 days going almost 8 hrs per use. That thing is still at 75% power. Super slim, lightweight, and has great power longevity. Product also comes with a handy AC charger AND a car charger. How can you go wrong? Just follow that installation guide in the post, and you can put that thing anywhere!