You just have to let it cool over the form and it stays rigid. Actually, it is a bit stronger after the heat forming because it relieves stresses that might develop in original casting.
Nice work on the bowl, @davidaspitzer. I have some colored acrylic and it would work great. These types of acrylic bowls really could come in handy. I have thought about making small ones that can be personalized for a dessert course at a dinner.
Here is my foray into it. I used 300 degrees.
By the way, my bowls are two years old and get regular use with fruit and bread. They haven’t cracked or broken.