Instead of a release sheet and covering mat board, I've been using a piece of foam on the bottom and a large sheet of aluminum foil on top. The aluminum foil is disposable, in case it gets wrinkled or dirty, but it acts as a good release sheet and conducts heat well. At first I was wary of this scheme, but I've been using it for about 10 years now without any funny attributes transferred to the prints. It is important to keep everything flat, even, and clean.