Hard, flat, and heavy? Sounds like a back rivet plate. Mine's a chunk of cutoff scrap from the remnant room, 1/2" thick by 8 by 20 or so. I grabbed a cheap set of angle grinder grind/polish wheels from HF to make one side pretty. It works great. Only thing I'd do different now is get cold rolled instead of hot rolled, because hot comes with a lot of mill scale.