We use conveyor belts at the rod and gun club, now. Before, 2x4's were held on rails and targets stapled top and bottom, with no backers behind the target. At Hefley, the Alberta Bullseye shoot has targets stapled to plywood.
For the chunk match, I use 4'x8' coroplast (plastic sign board) - 4mm thickness, cut into 2' X 8' strips. The targets are stapled to that, which seems to be working and holding up just fine. It is in good shape for this coming match as well - the 3rd such with the same backing.