I've heard of people using cleaned styrofoam from meat trays but didn't have any on hand. Looking around I found a bunch of old MRI films from when The Young One injured her shoulder at karate. I've used a couple of them to cover the work table while doing messy work. They were thin and would do the trick.
I traced the palette with a black marker, cut, and covered. An elastic band to hold the cover in place will be perfect.