That idea of a couple 2x8's and threaded rod isn't a bad one. The difference is to use carboard instead. I made one like this many years ago. Took large carboard boxes and cut the carboard on 12" wide strips. Then layered them and pulled it all together with 1/2" threaded rod. The end result was a 4'x4' target backstop that was almost as hard as a piece of wood. I could put multiple target faces on it and move them around. When ever it started getting soft spots from repeated shooting I just loosened the rods and moved the sheets of cardboard around.
Mounted the thing on wheels and could roll it in and out of the driveway or where ever I wanted. I think it lasted me almost 10 years. Not good for broadheads though. Total cost involved the threaded rod, washers and nut, 4 wheels snd axles (look for an old kid's wagon).
Now where would you get enough boxes? Try the dumpsters behind stores in a mall. Ask at any of the stores such as Bass Pro, Gander Mt, or other dept. stores. You'd be amazed at how many of them they tear down and band together.
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