This is the second version of the sandbox trap that I've made, trying to make it smaller and lighter. This seems to work very nicely and is quite portable (bail out sand and put it in wheelbarrow). Can also be moved easily with basically any tractor loader/skid loader or even a 3-point hitch if you buy a couple pins for it. These of course still need a proper backstop.
I use old 1/2 plywood used for concrete forms and whatnot for the face.
Unfortunately a 2x8 is not quite thick enough. Heavyweight 44 SWCs at LOW velocities like 950 FPS like skeeterload will penetrate, while faster loads like the Keith load will not.
M855 perpetrators, M2 30-06 ball, and 9mm ammo don't get though interestingly. I borrowed the fine masons sand from my kids' sandbox.
If I were to make it again I would use a 2x10 or 2x12 side and equip it to be carried by a tractor's 3-point hitch.
Fine sand weighs about 100 lbs per cubic foot. So keep in mind how sturdy the carpentry needs to be if you scale it up. These are so cheap to make (lumber was a salvaged deck mostly) that you can afford to make one for each alloy you use and one for jackets. This makes it so recovery is very easy...basically infinite recycling of bullets.