Fella's;
I've built several of my own design, and they're permanent. I use 3/8" rebar, though if I were to build another it'd probably be 1/2", and have it bent into a flat-bottom U two feet in width. The legs extend down as far as you want, buy the rebar to length, about 12 feet in most cases I'd think. Acquire 6 feet of 2" heavy angle iron, cut into two 3' lengths, & bore holes a sixteenth larger than rebar diameter in both pieces 2 feet apart.
To assemble, put the upper angle iron with the lip up into the U, weld to legs, & you should have a 2-foot square target frame. At the bottom weld the angle iron lip down about 8" from the bottom of the legs. The lower stiffens the legs & provides a pair of pounding points should the ground be hard.
Common office bulldog clips then retain the cardboard background material to the frame & staple the target to the cardboard. These can withstand some rifle round hits & most common pistol round hits. If the legs are long, they can be a PIA to lug around, but they aren't heavy, just cumbersome. I'm fortunate enough to have my own range & just drive them out to distance in the pickup & set them up. Most of the time the 3/8" rod can just be stomped in with foot pressure, but when the ground is dry, half-inch legs are better for hammering them in. The heavier duty the angle iron is, the better for two reasons. One, if it's hit by a bullet it doesn't deform as much. And two, if you do have to pound the target in, the beefier angle iron holds up better.
Why 12' of rebar? Because two feet at the top of the frame leaves two 5-foot legs. The bottom of the frame, 2-foot square remember, then leaves the legs three feet long below the frame square. That's a good overall height in most cases.
900F