Natural rubber is your best bet for staying flat, but check the thickness.

Natural rubber stays flatter over time than silicone or polyurethane, assuming the pad is at least 3mm thick and properly cured. The manufacturing process matters: cheap rubber pads are sometimes under-vulcanized, leaving residual tack that makes edges curl during shipping. Let them sit under a book for a day and they usually flatten out. Silicone bases are almost immune to curling because they’re molded in one piece, but they have a different feel—grippier, less natural glide. Polyurethane foam bases (common in “desk mats” and gaming pads) curl most often because the top textile layer and foam expand