http://erinerickson.com/2011/11/circ...ouch-tutorial/
Instead of a zipper, you could overlap the opening (like when doing the back of a pillow cover). You could use those small velcro dots, or snaps to close the opening. They could hand sew those on. You could also make it square. It might be easier for those that might not have sewn before. No matter what you make, it sounds like fun!! Good luck.