My pits are about the same dimension as yours, and I have two of them with 1 1/2" fiberglass. I reinforced them with two pieces of angle iron across the 40" span. I've pulled my 3/4 ton pickup onto them and they don't bow at all. I purchased them from a company called Versa-Grate several years ago when the metal ones were getting stolen. They had a good price and service, but I don't know if they are still around.