When vac forming a canopy,
be sure to extend the base with a skirt as mentioned above to allow you to cut very close and trim to suit 👍
1. quickest option would be a block of balsa (or soft wood) and carve it to the shape your looking for 👍 it will hold great under vac form pressure
2. Cad one up and print it but you will need to fill, sand and polish to remove layer lines that will be pulled into the vac-form.
printing the plug 5 walls are enough and 0% infil is best , while printing at best resolution, 0 bottom layers and once printed fill will plaster which will ensure no collapse under the heat and pressure of vac forming.
sanding and polishing - simple paste filler or rattle can primer filler is all that’s required, but be sure to spend time filling/sanding/repeat then final polish to get the best result you can
another tip for a clean smooth finish it to heat shrink plastic or vinyl over the plug before vac forming
the above can be done with empty clear plastic drink bottles also with. A little practise 👍