view from inside
Opening wide the maw of the vacuum former, the two plates with rubber gaskets (white squares) can be seen on top and bottom. Usually, the 12" gaskets are in place, and they will need to be removed to make a 24" mold.
bottom gasket
Remove the bottom plate by grabbing on the inside of the open square and pulling up. The frame will lift a couple inches, then run into catches on the front corners. Work around these until it can be removed. Notice that there is another seal underneath that will act as the gasket for a larger sheet of plastic.
top gasket
To remove the top gasket, open the frame, slide one hand through the open square from underneath to grab the frame as shown, and keep the other hand on the outside edge of the frame. Pull straight out. This one tends to be a bit sticky to remove. Frames can be replaced in the same manner they were removed.