Best info on steel sheet for KeumBoo

Is there a best steel sheet/block for keumboo? Currently researching cold rolled sheet steel. It makes sense that it should be large enough to extend just over the coils so things won’t fall between, and I’m pondering 14 gauge, .064, 1/16 would be better-less likely to warp, than thinner gauges. Understanding it will blue up within the range of temps best for successful bonding I ask if anyone has had good or best experiences with a different gauge or suggestions on size or shape. (A round plate would be awesome, or not?). Thank you!