cheaper to just go to micheals and buy some 6mm foam, spray it with adhesive, then cut the slots.
The Art Bin boxes are the exact same as the Cliffs boxes except for color and sell for $12 ($6 on sale) and the foam is $1.
Here is a picture of a Cliff's "The Beast" for comparison. You can see they are the same thing other than color. Even the foam is the same 6 mm thick stuff that has been sliced.