2 x6 lumber- faced with 1/2 inch to 3/4 ply, OSB, whatever. I line the target side with a couple players of cardboard. My theory is that it helps keep the sand from running out the bullet holes.
make a box with the 2 x 6, use screws. ( mine is 2 foot x 2 foot) leave the top 2x6 unscrewed. Place target box where you want it. Fill with dry sand and put the lid on. Leave a small gap so the sand has somewhere to go when a bullet hits. If it does not have a tiny bit of room to move, it could blow out a side or the top. Fasten target and shoot away. When you need to make more projectiles, sift the sand outside and remelt the lead.
I have shot the same couple of pounds of lead for about 4 years, maybe more. The only projectile to go thru and thru was an 7.62 FMJ from an unmentionable, but it did stop 4 of the 5.
Replace the target sides before they lose structural integrity
use with a proper backstop- banked rock free sand, earth what have you