When I am making a foam wing and want to sheet the inside with balsa.....here is how I do it. First off, cut out the foam to the right size. I use a tuna fish can attatched on a drill with the edges sharpened. I make up the required balsa to completely cover the inside of the wheel well. I usually make it oversize. Use white glue also. Then I wet both sides with Windex spray. Gently roll the balsa into a tube and fit it to the foam wells. Then I get a balloon from Wendys or any balloon will work. Blow it up while it is inside the well. I usually make enough pressure to make a dumbell looking balloon. Tie the balloon off and let it sit overnight. The next day the balsa is nice and round. I deflate the balloon and trim the balsa so it is flush with each end. Next use glue, epoxy or white glue and coat the back side of the balsa and the inside of the foam , install the balsa and do the balloon trick again. It presses evenly all sides and makes the balsa stick very well. I usually make the wheel well go thru the entire wing. After the glue dries remove the balloon and trim off the top and bottom excess. You can sheet the top and bottom of the wing in your usual manner.