RE: CG Chipmunk mods
Wing tips: Using the plastic wing tip as a template, trace out the overall shape of the tip onto a sheet of 1/32" ply. Then stack/glue a few 1/4" thick balsa pieces onto the ply part to generate the thickness of the tip. Now hopefully you have a small disk sander - Otherwise, well, it's a bit more work I suppose. Anyway first get the airfoil shape of the tip, then with the balsa tip and plastic tip side by side on the bench, carefully try to mark the front to back curvature of the plastic tip onto the wood tip. The ply part will get glued to the wing. Using ply stiffens the trailing edge - otherwise balsa alone will break.
By using a stack of 1/4" parts, you can use the visible lines at the glue joints to help create a consistent/symmetrical tip. When done w/ the shaping, use a dremel tool to hollow out the inside to save a little weight.
Belly Pan: Mount the wing to the fues with some wax paper at the fwd and rear fues. Actually this was pretty simple to build a box to conform to a belly pan. Did require some careful fitting, but once done, looks much better than the plastic. Just need to plan out how you're going to work around the wing attach bolts.
Hope that helps. . . .