If your model has any space under it, the plastic will form under the model and make it impossible to release. One way to prevent this is to place your model on a bit of wood and use modeling clay to block the bottom edge of the model.
I have tried both water based, air drying clay (the kind you might use to make a pot) and an oil based, never hardens modeling clay.
The never harden clay melted just a bit with the vacuum forming and was a mess to clean up. I placed the clay more as a "rope" around the model and that left a lot of nooks and crannies in the formed plastic that clay was stuck in. On one cycle, for some reason, I smoothed out the clay with my finger and that made the clean up much easier. Still, it is a pain and I don't recommend it.
The water based clay seems like it's going to dry on everything and be a mess. In fact it is easier to clean up. The clay did not "melt a bit" in the heat of forming; I think it became a bit harder. Then the clean up is easy with just flooding the formed plastic with water.
I wish I had never thought to try the oil based clay!