Wrap them in plastic wrap.
Thanks. I'm scared it will make ridges though. I'm not a master at keeping cling wrap smooth :)
You can get the large sipping sized roles. They are 2 feet wide and the plastic is thicker. Any ridges you get will be small. With a bit of touch up work after cutting the cast off the person can smooth that out for you. Either do some light sanding to the inside or rub in some additional plaster to smooth things out.
sanding will make the cast a TINY bit larger unless you are very careful, adding plaster will make it smaller. Things to keep in mind.