My solution to this was found at Lowes. They have a nice white poly...? foam normaly used as a spacer between large glass bricks. This is neat stuff. It's the right thickness and it won't take a permanent set, you don't have to buy a big sheet and it's cheap. Three layers adds up to exactly the distance between my bottom skin and the cover. This stuff cuts easily with sizzors and glues quickly with contact cement. Cut your top and bottom then the spacers around the tubing. Glue this up and you're good to go. Have fun.
Bob Whisler (Lowes has it, HD does not)
RV-7A, finish
Dayton, OH