When I make the time, I'm going to make one.
My idea is a 55 gallon drum on it's side, supported off the ground.
At the middle of the top of the drum (layed on it's side) cut out a square section. No larger than the two middle bump outs that go round the drum, maybe that size in a square.
Take some thick flexable rubber cut just large enough in a circle to fit in the end on the inside. I'm thinking 2-3. inches of rubber overall. Work it in and place it against the drum end. The end without the two bung holes.
Fill the drum up at this point with dirt or sand, while it is horizontal, with the blank end (bottom) towards your firing line. After you place it on the supports. I've got old power poles I will use.
Take another 55 gal drum and cut out enough of the middle to cover the hole cut in the first one, overlapping about 2-3 inches. Attach it with heavy sheetmetal screws.
I feel the rubber will help to slow the sand/dirt loss, as it will somewhat self seal after the bullet passes. The cover on top will keep out water. And the drum itself will keep the lead in.
The only drawbacks I can see, is it's a small target area. And after awhile the rubber will be gone.
At that point, I figured to cut out an opening the whole length of the top of the drum as wide as the original hole. Work in some barriers every 1/3 of the length. Cut a new drum up and make a hinged cover for the top. Then seal and refill as needed.
Might do that from the git go. We are emptying some drums this week, I've got my rubber, now if I can just find that bucket of motivation... I know it's around here somewhere...