Free, really, if your gun nuttery is comorbid with a chemical dependency on Corn Flakes.
Yes, just hack up the box. It seems cheap cereal box cardboard allows for nice holes without tearing the way copy paper does. Wadcutters, like diabolo airgun pellets, work particularly well. The targets may need frequent changing, because the centers tend to fall out of tight groups, but for zero outlay, I'm not complaining.