Make a good, cheap, upgradeable sheet plastic vacuum former

Step 7 Make a removable weatherstrip gasket

Now make the gasket for your chosen frame size. You want it easily removable, so that you can use different-sized plastic and frames for different projects, so don't just stick the weatherstrip right down on the platen.

Instead, use a thin sheet of flexible material, such as 1/32" plastic, maybe the same stuff that you'll be vacuum forming. (You might think it would stick, but it won't.) One of those cheap flexible "garage sale" signs from a hardware store will do fine, too.

You'll put the gasket on this "tape-down sheet," and tape the sheet to the platen with masking tape or painter's tape over the edges. (You might think this would make a lousy seal, but it works fine; vacuum sucks the tape in so that it seals better. Positive pressure would blow it right off.)

Cut the plastic 1/2" or 1" bigger all around than your frame size---only a half inch if it's almost the size of your board (or the plastic sheet you're cutting it from), but an inch otherwise. An inch is nice, but a half inch will do fine. You don't want it way oversized, because that just makes a bigger edge you have to seal with tape.

Now cut a hole in the middle so that your tape-down sheet won't block the vacuum hole in the middle of the platen.

(If you're applying this technique to a many-hole platen, which you can also do, the tape-down sheet should be big enough to cover all the holes, and the hole in the middle should be big enough to expose most of the holes inside the gasket.)

Mark the rectangle where your frame will meet your gasket, both the inside and the outside edges. That's where you want the gasket.

I like to make mitered corners in my gaskets, cutting a 90-degree vee out of the material at the corners, but not quite all the way through---leave about 1/8" of foam at the outside edge, rather than cutting all the way through it. Other people cut theirs square.

The weatherstrip is flexible and stretchable, which can make it difficult to apply neatly, evenly, and in a straight line, if you peel the backing tape off of it too soon. So don't. Peel the backing paper off a few inches at a time, and carefully smooth it down without stretching it. (There should be some slack.)

That's especially important at the corners. Don't cut your pieces too short, or cut your vee-notches too soon. Lay the stuff down almost to the corner, and then cut it a shade too long, maybe 1/16", rather than a little too short---lay it down slightly scrunched at the corners, so that the foam presses against the joint and holds it closed, rather than being stretched and having the joint gape open.

(Don't obsess about this, either, though---if you get it wrong, you can fill the gap with silicone, or re-do it, and it will be fine.)

If you're in a hurry, don't bother to seal the corner joints. You can go back and silicone them later, when you won't be using the thing overnight. (Or use rubber cement, which sets up quickly.)

You probably don't want to actually tape the sheet to the platen at this point; wait until you've adapted your vacuum hose. If you do tape it down, be careful not to damage the gasket when you're fiddling around with the bottom side of the board.