I don't do vacu-form per se, but squish form which is fast and almost as good for canopies.
First you cut a shape in plywood (i.e. the width/length of the bottom of the canopy), and carve+sand a balsa former:
Then you clamp some 'easter egg packet' plastic sheet to the ply frame. Heat under grill or heat stripping gun, and just as it starts to wilt you ram the former through the hole. After it all cools down you just cut the canopy from the plastic sheet.
I recently tried using the paper canopy from a paper plan to shape milliput into the canopy shape (while sticking a cocktail stick into the back of the blob for later). When it dries you have the canopy former with a stick to hold it by