I have my kids collect styrofoam trays from the cafeteria. For a weekend, I
use a fine mist spray bottle and spray the surface of the paint, then cover
it with Saran Wrap. I have found that it works better than the other plastic
wraps (it should, it costs more!) I basically press the wrap into the paint
at the edges to form a seal. Some paint is wasted when lifting the Saran,
but one can always lift the paint from the wrap with a brush to avoid as
much waste as possible.
Also, when I was in school, we would put a small, dampened piece of
sponge on the plastic palette before covering. That worked pretty well, too.
Linda in NC