I've found a handy use for those Corex (corrugated plastic) lawn signs. They're really good at holding carriage bolts so you can paint the bolt head. Even better than cardboard-- once you cut an "X" in it you can use it again and again with different sized bolts. I'd offer to share some of mine, but you probably have your own local source.
I use certain ones for target practice...
I suppose that's another way to make the holes for the bolts, but my aim is not good enough to lay out a grid like this. Carpet knife is more precise and quieter.
Great idea Arthur.
I hadn't thought of using them for target backers. That's a great idea. I don't have any, but could probably get my hands on some.