RE: Vacuum forming questions
Balsa will not work well. I have vacuformed several canopies using balsa molds that were finished with about 20 - 25 coats of clear dope. The point of the clear dope was to build up a surface that could be sanded lightly to remove the wood grain.
I still had some grain showing in the clear canopy and the dope melted slightly under the heat of the plastic. You definitely need a hard surfaced mold that will stand up to the heat of the hot plastic.