I use a layer of saran wrap between the bridge and plastic to keep the wood dry and to be sure it won't stick. But maybe it's not necessary after all
Anyway, set the bridge on the bench, saran wrap over the bridge, molten plastic on that, and smush it down with something flat. After it cools, peel the saran wrap off and saw/chisel the excess plastic from around the bridge imprint so you have a nicely fitted caul that only contacts the bridge and not the soundboard. Use a flat block of wood to support it when clamping. The plastic is only to fit it to the bridge. Not stiff enough to use as a caul by itself.
