This is what I did in my last kayak that didn't have a battery bag and I'll probably do it on my OKT15 I don't think I'll end up using my battery bag because they can't hold a watertight box in them
http://mobile.walmart.com/ip/26674163
Check out this item I found on eBay:
http://pages.ebay.com/link/?nav=item...225261&alt=web
This box with a cable gland and I zip tide it to the scupper post
This is the only pic I had of my last one but the battery is usually on its side with foam to keep it all tight in there

hope this helps