In my old darkroom days, I used stacked fiberglass window screens for drying. Available in many sizes, just get what you need. I put 1/2" or so rubber bumpers on the corners to space them a bit. They really worked very well. I have recently made some storage boxes out of glued form core. Make it so the prints stack vertically. Adding a few more dividers than I did would accomplish what you want. Can be made in any size and you can even make a hinged top to keep them clean. Very easy to do, all you need is some foam core, a good ruler, a sharp blade, and some foam core glue.