Any oven stone will work. If too large you can cut it with a tile or masonry blade quite easily.
I have bought a 1/2" thick travertine tile quite cheap and cut it so there was about 1" around all edges.
Using some kinda stone like this allows for the BBQ to be used like an oven. We have done mac & cheese, cookies, squares, lasagne, meat loaf and other things.
2017 Escape 5.0 TA
2015 Ford F150 Lariat 3.5L EcoBoost
2009 Escape 19 (previous)
“Most folks are about as happy as they make up their minds to be.” — Abraham Lincoln