I've seen everything from metal flashing to cement collars for this type of thing.
Do you have an IR temp gauge by any chance? If you can get a temp reading of the surface of the wood, you can get an idea of what is effective. Some metal flashing that extends down below the wood around the bowl would probably work well. The additional surface area of the flashing will help dissipate heat, I would think. But for longer cooks, if the area around the flashing is still getting too warm, a nomex gasket between the flashing and wood might be all you need.