Umm... doesn't plaster get rather hot as it sets? In sufficient quantity, it might get hot enough to deform the vacuform part...

Sit the canopy in ice water just to the brim.

It may take the plaster a bit longer to set (cure) but not much.

We're talking about less than one oz. of plaster to fill the canopy. It won't get that hot.

Epoxy does the same thing when it cures.

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I wouldn't fill the canopy with epoxy if I were you.

Of course, you could always use something to act as a barrier so that it doesn't stick.