These are very simple, and inexpensive, but I think they are pretty cute, too. Just a piece of posterboard folded in half (or two full sheets of posterboard for a larger case-like the ones I use for my kids' keepers at home). I used red duct tape, because who doesn't loved colored duct tape? And hole punched the top the case, where I tied red grosgrain ribbon for handles.
I think I've shared this site before, but I love Art Projects for Kids, so I'm giving you a link again.
How do you encourage little artists?