I've been stacking mine with sheets of YUPO polypropylene watercolor paper between each plate. I use half of a 9x12 sheet for 5x7in plates. YUPO is a little on the expensive side, but it's tough, reusable, and smooth enough I know it won't abrade the emulsion. Make sure, of course, that the plates are good and dry before stacking. Keep the box small enough that the plates can't knock around too much.
Best of luck,
d |