Hook up the vacuum hose and cut a piece of plastic 18"x13 1/2" place it in between the two frames and clip them together with those big heavy duty black binder clips; 4 on the long sides and 2 on the short sides. Make sure that they are positioned so that they do not interfere with the plastic contacting the edge of the cookie sheet, it will take a bit of getting used to. Raise up the frames 1" under the grill and hold them in place with additional binder clips. Place your form on the vacuum table. Turn on the grill, the plastic will heat up and sag, when it has drooped about an inch to inch and a half it is ready. Remove the clips holding the frame up and drop them to the side; quickly and steadily lower the plastic onto the the form and apply moderate pressure to help from a seal. With your knee (or assistant) flip the the socket switch. The vacuum will suck down the plastic. After about 3 seconds, I turn off the grill and the shop vac will surge causing additional suction. While maintaining pressure, I leave the vacuum on for about 20 seconds so that the plastic cools a bit and does not lose it's shape.
Notes: I love my machine and have made teeth, eyes, faces, masks, underskulls, packaging, finger nails, hand forms etc. You can vacuum form many plastics, the easiest is styrene in thickness up to 1/4" I prefer .020 to . 060 PETG is clear and flexible, ABS is what Fieros were made of and I used it to make Marilyn Manson's 16 foot tall gun cross for his world tour very strong stuff. Sometimes you will have to drill holes through your bucks for better detail.
Bucks are the positive forms that the plastic is formed over. You can use plaster, urethanes, epoxies, wood (sealed), silicone skins backed with rigid foam, or rigid foam skinned with thin vacuum formed plastic for your bucks. You will need to heat the plastic more or less depending upon the buck,. Watch out for undercuts, you can do a full head or skull just do it in two halves, make a mold of the front and back; cast up some bucks and go. You can make ABS armor that will be ten times as strong as the stuff that you can buy and it will be light. You will be amazed at what you can do. I will take some photos of my machine and attempt to post them ( I am pretty lame with this stuff) I hope that this makes sense, I am the kind of guy that just grabs stuff and builds.