Ferric chloride will etch brass and copper, I use brass because it's a bit cheaper. 20 gauge is plenty thick, and it's available at many hardware and hobby stores, and of course online. KC Engineering makes it in many sizes. After I cut it with my jeweler's saw, I clean it with Penny Brite copper cleaner and 220 grit sandpaper. Comet cleanser also works for this, just not quite as well. Clean metal is critical, water should sheet off the surface. If it beads up it's not clean enough yet. Dry the clean metal with a clean cloth or paper towel.