Hey Hangfire (great name, by the way),
Don't bother with steel (no offense intended, Iron Dog). There is an easier way. I didn't believe it when I first heard about it.
Make a plywood box (stay with me here) about three feet long, two feet wide by two feet high. Give it a hinged top. Make one of the two by two ends so you can change out that end easily. Maybe 1/4" plywood in slots cut into 2x4 like a picture frame. I use those sheet metal angles for holding stair treads in place.
Make legs to prop it a couple feet off the ground. Fill it with rubber playground mulch. It's just chipped up tires. You can get it in bags at Home Depot or any yard supply store.
You can shoot a 62 caliber ball in the end and it will stop about 18" deep. I put a 1/4" steel plate in the back end just in case, but I get about four dents a year in that. That only happens if I let the mulch level go down and shoot high.
Eventually you can either scoop out some mulch and poke around in the box, or else dump it all out on a tarp and pick through for your lead. The balls will retain about 99% of their mass.
I have noticed that with a particular ball and load they will all cluster in the same area of mulch. I can find them by the stain of sawdust from the front plywood panel.
When the front panel gets too shot out you tip the thing onto its back end and slide out the old, slide in the new.
I just made a new one which will have a sloped top and tin roofing. The old one rotted out after a while.
Happy shooting.