Here's a tip--
In our school the "bread man" delivers bread on many stackable
20" x 24 "plastic "trays"--that have open-grid bottoms & sides
with cut-outs for carrying.
All sizes of slabs and tiles dry beautifully on them.
Throw a loose piece of plastic over the top, forget about them for a
few days, and you're done--no warping.
If Sax could find out where the bread guys get them, and keep the
price low ( don't call it an art product), they would serve many purposes
in the art room. They only take up a small piece of the floor ( or
and you can stack a few, or a dozen. Love those bread guys.