We use the 5x8 foam pizza trays from the cafeteria. We buy them by the
sleeve. We use masking tape to hinge two together, which opens to make
two concave areas on which to mix. Acrylics rarely drip down from the
lid onto the lower piece. Between classes these are shut, placed in a
plastic bag with a damp paper towel, and last for weeks with fresh
acrylic.
Beth