Went to my buddys welding shop and scrounged up a piece of metal from his scrap pile. 1" thick high stregth steel part from a fork lift attachment. Brought it home and cut it into a 5" X 22" plate for back-riveting using the CNC with a flame cutting torch. Weighs about 30 Lbs. Overkill I know, but should stay put when riveting... And, with the steel left, i plan to make custom shaped bucking bars whenever they are needed for hard to reach area's. I cleaned and smothed the plate surface with a sander, and then used the CNC router with a Scotchbright pad to "Polish" the surface.