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.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver