Ken, I built a Coho and put a minicell bulkhead right behind the wood reinforcing strips under the deck. That gave about 3" which is plenty of room. Having a minicell seat extending beyond the hip braces to to side panels and back to the bulkhead is a good way to displace a couple gallons of water to the front of your seat for removal. Putting the bulkhead further back like some plastic boats is not a good idea, you can't get to stuff without getting out of the boat and it just allows another cubic foot of water in the boat. If you are making a CLC boat then glassing the under side of the deck before and after the installation of the deck in that area is a good idea. What kind of kayak are you making? there are more convenient places to affix little stuff without putting it where you can't get to it conveniently. Under the deck, on the floor between your legs(bungied down), afixed to the back of the backband, behind the hip plates, etc. Just checked my Mariner, the minicell bulkhead I put into it is about 4" behind the coaming, that's enough room for two 2 1/2 gal collapsible bags of water, that's 40lbs of buoyancy I don't have when flooded although I liked being able to unload two heavy things quickly. A small Pelican box