One of my favorite books on the subject is
"The Prop Builders Molding and Casting Handbook". In a prior life I did a lot of theatre work...
There is about a 50 page section on how to build and use a vacuum forming machine. If you have access to a college theatre department, they would be a good resource in the prop and scene building area.
One trick that I've been told by an expert that I know is to cover the mold with a rubber sheet dusted with I think talcum powder. I'm away from the office all week, I'll have to try and find my notes when I get back.
Good Luck, post some pictures when you get it built!