Make a good, cheap, upgradeable sheet plastic vacuum former

Step 6 Clamping plastic

This step isn't really part of making the vacuum former, but you'll need to know how to clamp the plastic, so here goes:

Orient both frames so that the useless groove is on top, so that the side with the wider flat area is on the bottom. You may want to mark each frame on the top, so you don't forget. (Maybe "BOTTOM FRAME, THIS SIDE UP" and "TOP FRAME, THIS SIDE UP.")

Take a sheet cut to your frame size, put it between your two frames, and clamp the frames together around it with binder clips.

For most thin plastics, and sheets up to about 12 x 18 inches, 6 binder clips are sufficient---one at the middle of each shorter ends, and two spaced about 1/3 and 2/3 of the way along each of the the long sides. (For thick plastics, larger sheets, or difficult-to-form plastics like acrylic or polycarbonate, you may need more clamps. A clamp every few inches is generally enough.)

Because our clamps go on the outside of the frames, they can interfere with the seal between the bottom frame and the gasket if we're not careful. If they don't sit flat against the bottom, there can be a gap that the gasket doesn't seal, and if they're too far out, the rolled edge of the binder clip can hit the gasket, which may cause an air leak. (And if you do it a lot, you'll tear up your gasket.)

Look at the bottom of each clamp. Make sure the rolled edge sticks inward around the plastic about 1/4" past the frames, and the bottom is flat against the bottom of the bottom frame.

(Unless you plastic is just the right thickness, the clamps won't be quite flat on both top and the bottom; they'll usually taper in a bit, or for thick plastic they'll flare out. Just make sure they're flat against the bottom, and let the top do whatever it wants.)

Now remove the bent-wire handles from the binder clips, at least on the bottom, so that the handles won't get in the way of making a seal. (Just squeeze the handle so that the ends come inward away from the rolled edge of the binder clip at the ends, and pull it away.) Put those aside in a pile.

If you don't have a tight fit in your oven, you can leave the top handles on, flipped back and outward. If you do have a tight fit, remove them too, so they won't stick outward past the frames and be a problem.