[Warren]

That is close to the way we used to form Skylights. Only you would have the mold, similar to what you did. Then you heat the plastic in an oven. when it is pliable, after a few minutes, you take it out ( with gloves of course) and put it on the mold and clamp it down to cool. With skylights you would put it on a table with the form on it and use air to stretch the plastic to the height you want it and let it cool. You might find it forms better and more evenly if you try putting it in an oven for a few minutes instead of using the heat gun, which is hard to get an even heat all around it at the same time.