This is an easy box to make that will hold six panels. I make mine so they will hold two sizes, 6x8 and 8x10 or 9x12 and 12x16. But you can make them any size you need. The sides are made from door fascia available at Home Depot. You can see the six grooves are already there. The sides and bottom are made from door skin.
Cut the fascia 1/8 to 1/4 larger than the height of your panel, and the sides to fit the box with the panel easily sliding in the grooves. Drill pilot holes for small brads along one side of the box and attach that side. Flip it over and do the same on the opposite side. At this point, I slide two panels in and clamp the other side in place. This way I can make sure the panels will slide with ease. Drill your pilot holes and nail it together. The bottom is cut to size and nailed into place.
At this point you have a dry box. You may choose to make a lid for it, and paint it. Please let me know if you have any questions and how it works for you.