Make a good, cheap, upgradeable sheet plastic vacuum former

Step 9 Tape the gasket to the platen

Once your silicone or glue has set up, center the tape-down sheet holding the gasket on your board, and carefully tape it down with masking tape or painter's tape. 3/4" masking tape will do fine, but 1" wide painter's tape is nicer.

Apply the tape carefully to each edge, unrolling it and smoothing it on halfway on the tape-down sheet, and halfway off (on the platen). If you get a wrinkle, lift the tape up back to the wrinkle, pull it taut, and smooth it down again.