I take it you don't have a vacuum former or you wouldn't be asking this question.

The only way I know is to fake it. You carve a plug from balsa or another wood. Seal the wood well with multiple coats of lacquer or polyurethane. Let it dry for at least a week. You can put it in your oven to accelerate the drying, but the fumes won't make your next lasagna taste too good.

You could just leave it in the sun to bake as well.

Once it's dry, sand it smooth and wax it. Glue a dowel or two into the bottom of it and mount it on a board so that it's stable. the board should be big enough that it won't tip over when you pull the plastic over the form.

Next, build a frame to hold the plastic sheet. You can heat the plastic in the oven (carefully!) or get some friends with heat guns to help you out. Warm it up until it sags and immediately pull it over the form. Let it cool and pop it off.

Repeat until you get one that you like. It can take several attempts - especially if the canopy is deep or large. Good luck!