After discussing the different wood options, the best option for heat, gas, and not cracking is the Baltic Birch Plywood. I can get a 30" x 30" x 1/4" piece for a whopping $9.00 plus shipping. This will leave me plenty of material left for more changes in the future.

Note: I asked about the Marine Fir Plywood and his response was that the glues are better but the wood is not as hard and it would absorb more fuel. He suggested that I use a urethane coating on the Marine plywood.

The above "Cool Carb" parts appear to be very nice. Thank you for posting that information.

Thanks again for your replies.