Brett,
I had a fellow help me build one when I was a kid. We took a 55 gal. drum and cut a square hole in the side about 1/2 way up. We made the hole large enough so a piece of plate about 14"X14" would fit in it. He did the welding for me and welded the plate at about a 45° angle and put some braces on the bottom. Mine would stop anything up to a .22 mag. I shot it twice with a .38 but it put dents in the plate so I stopped. Filling the bottom with sand will also keep the rounds in the bottom once they hit.
One word of advice, if used inside they are loud no mater what.
Rusty <><