Make a good, cheap, upgradeable sheet plastic vacuum former

Step 4 Attach the floor flange to the platen board

Now you need to attach the floor flange to the bottom of the board, over the hole. Center the flange over the hole, and mark the spots where the screw holes are with a pencil.

Drill pilot holes into the board at the centers of those circles. (The pilot holes should be about the diameter of your screws' shafts, not including the threads, or a shade smaller.) Drill most of the way through the board, but not all the way.

Clean the surface of the flange where it meets the board, using soapy water and then non-soapy water. Galvanized fittings often have a thin protective layer of oil on them, which can keep sealants and glues from bonding well to them.

While you're at it, dampen the area around the hole just a little, maybe pressing a damp paper towel to it. (This will make silicone set up faster. You don't want it really wet, which may make the MDF swell; you just want a touch of moisture.)

Put a bead of silicone caulk or silicone sealant around where the flange meets the board, going on both sides of each screw hole.

Press the flange to the board, and screw it down. Wipe around the edge of the flange with a paper towel or something to remove excess silicone.

(You can use epoxy instead of silicone, if you're in a hurry, or if that's what you have handy. In that case, don't dampen the MDF, but do make sure the flange isn't oily.)