I made this out of leftover masonite after having a sheet cut for canvas panels. That's why the ends are longer than the sides. A carpenter, I ain't!
Anyway, it was cheap and easy.
I had the lumber co. man cut these wooden strips 12" long (they came in 36" lengths). I glued them onto two 12" high pieces of masonite, then attached them to a bottom piece. I glued the pieces together and then used L-shaped pieces of metal with holes for screws (whatever you call those things) to screw them together. Attached the end pieces the same way. The picture shows the box with several masonite canvas panels inserted between the wooden strips. It holds them apart so you can carry them without smudging.
Now I just have to rig up a handle, and it will be great.
I saw something like this for sale on a website, but this was real cheap and works fine. I made it big enough to carry a dozen or so panels, but it could easily be made to carry 3 or 4.