I built a set of plates and a rack, years ago for my home range. No plans but I can give you some pointers. I built it, tore it apart and built it again, about 6 times. Figure out the size of plate you want, 10 inch, 8, 6 etc. For what I am going to describe you need a piece of angle with one lip at least 1 inch tall for the front or you can use a flat welded to the top flat, but make sure the front flat is set with the bottom in front of the top, if you weld the two pieces, this prevents bullet warping at the bottom or weld breaks. The uprights need to be solid, I used 3 inch pipe, otherwise use something big and heavy that can be set in the ground, like RR track.
Regarding the plates, the plates at 0 angle will produce splash back, so you must angle the plates forward or back at the top. A free standing plate requires about a 1 1/2 inch square to sit upon, however a hinged plate is much easier to work with and must be angled forward to function correctly. You simply weld a stop on the front side, bottom of the plate, then weld half of a steel door hinge on the plate bottom with female portion to the rear, a small block is welded on the rack top to raise the male portion of the hinge to match and the other half of the door hinge on the plate. I welded the male portion of the hinge on the rack. The plate looks something like this from the side _\. the connection on the rack looks something like this from the front =- The front flange protects the hinge and the hinge is mounted on the top rear edge of the top flat of the rack. The forward cant can be as little as 2x thickness of the plate at the top.
Everything is based on an 8 ft length. Set the center of the end plates in 9 inches and each plate 18 inches apart, at the center. The plates should be 3/8 to 1/2 inch thick to prevent warping.
If you weld a flange on the back of the rack uprights, below the top, with a hole and a rod for a long hinge with a second flange and another rod, one of the second flanges is a long lever and a rope so the plates can be set by pulling the rope. The rope end of the lever must be higher than dead center so you will need a stop to prevent over travel of the long lever. Galvanized pipe can be used for the rods.
I hope this helps,