We have a ventless isokern installed (new construction) but want the "surround" to be around the opening about 3-5" out. However, due to code, we need to be like 20" from the top of the opening if it will protrude out 5" if it is to be made of combustible material (wood frame). We are doing a plaster (venetian plaster) around it so trying to figure what to build the surround out of. I imagine cast stone is expensive and not sure where to find this. We want the fireplace to look "recessed" like the last picture. (Ignore the blue lines). Any suggestions?