Step 4: Prep pan
Fill your bags about 1/4 full with pea gravel, beans, rice, or other similarly-weighted items. Tape or ziplock bags shut; don't tie them, or you won't get proper coverage.
Arrange them flat along the base of you pan, then create a couple of channels running the length of the pan, each about 1/3 of the way in from the sides.
Cover with a layer of aluminum foil or waxed paper (optional) then a layer of plastic wrap.
Tape the plastic wrap in place, especially on the edges to prevent jello sloshing over and getting under the plastic wrap. Make sure you've left enough flex to follow the contours of your gravel bed!