I love this thread because I desperately need to find the heat/moisture barrier. I have plywood with an old piece of plexiglass on top and have to keep stopping to let the plexi flatten back down.
For the very top, I use several layers of thick, white denim - just sitting there loose - from Joann's some years ago. It has a lot of grip when you're pressing, and it doesn't seem to go anywhere.
It's easy to turn one piece over if it gets grungy. Or wash/dry it. But if you have 3 pieces, that's 6 sides.
hugs,
charlotte