A fire resistant BBQ mat does not have to be rigid, although some are. I have purchased rigid mats. They are heavy and awkward to transport or move on the desk or patio. I currently am using one of the thin rubber mats under my Traeger. The come rolled into a tube. As long as you place them under your Traeger with the curl side down, they will lay flat. I live in the Chicago area where it can get quite windy. I have no issue with the mat curling upward.
If you insist on a rigid mat, one inexpensive option is to purchase a 3 ft x 5 ft tile backer board such as Durock, Fiberrock, or HardieBacker. Because these are cement boards, they are fireproof and waterproof. However, they are not particularly attractive. You can paint them or do what I did for my gas griddle and cover the backer board with tile and grout. That gets really heavy.
