Mild steel may work, but it will pock mark, and deform over time leading to potentially dangerous ricochets. A properly made popper will last a lifetime for an average individual.
If you are on a budget check out E-Bay. There are a number of folks selling round gongs for very reasonable prices. You can get a 10" diameter 3/8" thick AR500 plate for a bit over $50 with shipping.
Last edited by Bernieb90; February 22, 2011 at 02:04 AM.