Try getting a hold of a couple saw horses, and some 1/4" steel scrap metal pieces, about 6" wide x 16" long. Google "MY AREA scrap yard" or checking the yellow pages should yield some results.
Next, go to home depot and buy some heavy duty door hinges and some 1/4" carriage bolts & nuts. Drilling through the steel will take some time, but the wooden saw horses should just take a minute.
Fasten the steel plates so that they are hanging below the cross bar on the saw horses, set it up, and start shooting. The added benefit is since the plate(s) swing to the rear, most rounds are deflected downward.
You can even tape a target to the front of them.
**Note: a .22LR won't do to much to the saw horse if you hit the cross bar, but larger calibers and/or hollow points, etc., can make short work of a 2x4.