Here's a rough sketch, I could have probably gotten by with thinner steel than the 1/2 plate, but it was available free. However it is HEAVY, guestimate around 240 pounds!
I cut full sheets of available plywood into squares and replace as necessary to staple targets to.
After a while, I just clean all the lead and plywood pieces out into a 5 gallon bucket, and rinse with the hose until most/all the wood particles float out.
A few pieces of lead find their way out after the board is shot-up and needs replaced, most less than 18" and all their energy is expended anyway, only once did I actually have one come back close to me at 10 yards, and it was barely able to make the trip.
Of course I do only shoot into it with 38 spl and 45 acp, No 357 or above, they MIGHT find their way back out with a little more velocity, and I don't wanna know!
Circa 1964: Rocky to Bullwinkle:
"But that trick never works!"
But Bullwinkle tries it anyway...again and again