I have a 24"x24" VFormer and I needed form some parts from 12"x12" sheets. I placed the mold on the platen, sealed off the unused platen holes w/ masking tape, sealed the part to the platen (w/ molding clay), and made a 2 piece "sheet carrier" from plywood that held the 12"x12" sheets in the center (the plywood carriers have a square 11.5"x11.5" hole in the center).
The sheet needed to be kept up from the standard plunge depth (the mold is a fiberglass mold and has a tall base), so I used a 2x4 to keep the frame from plunging fully and therefore the sheet was suspended in the wood carrier above the platen at the correct height to form the parts.
I should note that based on the nature of the mold, I had to drill holes in it for the air to flow through and into the platen underneath. In this case, the holes I had to drill were in an area that gets cut out. The mold can still be used for FGlass parts.
Hope this helps.