A good way to fuel proof the inside of the plane without a lot of weight being added is to spray the insides with polyurethane spray paint (clear of course) which you can purchase at Home Depot. Fuel proofs the trays and wood and will allow you to glue or add things later without trouble. And by all means epoxy the wings together on the top as well as the bottom. Nothing compares to seeing the wings come apart in mid air.