Heated and ready
Here you can see the simple wooden stand used to support the frame. Don't leave the stand in there between heatings to avoid setting it on fire. A metal one would avoid the problem. The plastic is heated and ready to form. Trial and error will tell you when it is right. A sag of a couple of inches in the middle is a good indicator. At first the plastic may shrink before it starts to soften and sag. The forming rig is kept close by as the plastic cools fairly quickly. You can get it too hot as well so judement is required.