Gene_N, the ones I have bought have been corelite. That is also what I have seen recommended by the "experts". I usually go for the 1/2" thick ones. I have never heard of an issue with them exploding from moisture but I'm sure it could happen. However if you store them inside and don't...
Find a ceramic shop and get a kiln shelf. You can find many different sizes and shapes ranging from square, rectangle, round, and half-round. Most I ever paid was $20 for a 16 inch round. These things will take a LOT of heat!