Though my uses arent so ambitious and grand as yours nowadays, I've been using and re-using the same big ingot of "cerro-safe" for about 30 ears now.
Great for backing up real thin parts, and can even be poured into wood cavities.
I once made a co2 charging adapter for an old Giffard using the valve threads as a mold ... seems dangerous, but it worked just fine for testing the gun.