I built a frame out of 3/4" PVC to fit the cardboard used on pallets of feed to protect the bags from pallet nails.
3 pieces of PVC about 4 feet long, 2 elbows, 2 pieces of 1/2" rebar about 18 inches long comprise the frame. I screw the cardboard on with little self-drilling screws.
Lay down the frame, drive rebar into the ground even with the legs, and tape on the targets. I use a small pipe wrench to pull the steel when done.
Takes longer to describe than to do - and is cheap to replace in the event of an errant shot. Stands up in a bit of a breeze, too.
