David,
I made that from a 2'x4' piece of diamond plate from Home Depot or Lowes. Glued that to a piece of 1/2" plywood and then put a 1x4 frame around the bottom to attach the caster to. Added a handle and it's done. Perfect size to set the lid or main chamber on to avoid grease stains on the deck or patio. Aluminum plate cleans off nice to.