I didn't read through all the responses here, so if I've repeated, I apologize....
I cast my own balls, so I try to recapture my lead.
A cardboard box stuffed with magazines (all oriented in the same directions, then shoot perpendicularly to the sheets - that is shoot through the covers) is perfectly capable of stopping at least 20 rounds of 0.535 roundballs moving at 1850 fps from 25 to 60 yards.
Of course, you'll want a real backstop as others here suggest to stop fliers, but the box of magazines is great. Also good if you don't want to keep dumping pounds and pounds of lead into the environment in one small location.
The cleanup is kind of messy with the magazines, but I just sort through the chaff, pulling out mushroomed RBs and fragments. I estimate I recover >90% of the lead in the magazine (some fragments get through) and >80% of rounds (because I do sometimes miss the box or a flier goes through a corner and doesn't get stopped).
I th ink some others here have mentioned welding a steel box to actually trap balls. If your range is permanent, this is o.k., but if your range is public or you would need to move the trap, not such a good idea because of the weight.