some people have done it under the wing tube i did mine on top. i had a problem with my wing bolts not tightening so i glued the bolt ,cut the heads off and got threaded spacers with cotter pin to hold the wings on much better now built the tray with 1/2 inch balsa then pinned then from the outside...pictures were midway through but youll get the idea..she balances perfect now full tank or empty...and you can hover or knife edge at low altitude and see how much gas you got left....