So I was going to save up and buy a big field target, but when deerchser got rid of a bunch of carpet, I got to thinking I'd build my own. Started looking around, and decided to just buy a replacement target bag; Morrel field cover, $20. Wanted a hard center for the bag, foam insulation; $9. Free carpet, nice. Bunch of old plywood and 2x4s plus $30 in hardware makes a nice outdoor storage cabinet.
The target is basically 3 layers of foam board wrapped in carpet, covered in a black garbage bag, and then stuffed into a target bag.