ORIGINAL: Mttvannest
I used light ply to sheet the bay where the wheels are. It is perfect for the job as said before.
Mscic, I'm not sure how you are using the deli tray? The bay where the wheel retracts is about 3" front to back, 5" wide, and 1/2 deep. Right now i'm just planning on leaving it hollow in that area. I was just worried that air would scoop in there and kinda of baloon the wing apart.
The foam boxes you get lots of places are usually about 3/32" thick foam. You can cut the flat parts out and use it like sheet balsa. Sometimes the corners are the right curve for special uses. Burger King used to serve Whoppers in a box that a certain part of the bottom made a good airscoop.
As for your wing scooping air, don't worry about that, but do consider that you really only have the back of the wheelwell to deal with. I fly off of grass. No way am I going to leave the rear of the well open to catch whatever junk the tires kick up. It takes so little time to stick a piece of balsa sheet in there. With the grain in the right direction, balsa will bend to fit the curve with no effort at all. And the foam from carryout boxes works good too. I'd use the balsa however, because it's grain greatly strengthens the wing in that area. The foam does a little of that, but even 1/16" balsa adds a bunch of strength.