I have tried a number of approaches to this problem and I still get a surface that is far from the nearly optically clear finish I require. To answer my own question, Pam cooking spray works pretty well as a relase agent. However, the surface is still poor. I then tried an all wood mold, sealed and highly polished. Still not satisfactory. My next thought is to make a mold where the plastic will not actually touch the surface where clarity is required. Imagine a slightly dished area where the air cannot be evacuated. This seems like it might produce the needed finish. Has anyone experimented with this?