I use a piece of 3/4 or 5/8 inch plywood - not a fancy grade and it is pretty rough (about 18x24) - left over from some construction around the house. I covered it with 3 layers of warm and natural then covered it with two layers of muslin. Never had a problem with the steam. I have used the same piece of wood for over 5 years now. I rip off the old muslin and put a new fresh piece of muslin on it about once a year.