I can’t claim that this is entirely my own idea. I got the inspiration from a display in a hardware store that was constructed this way. It was used to hold the long, thin bracket pieces used with adjustable shelving. I took the basic idea, but added wheels to make it portable.
As you can see from the pictures, it is basically a plywood box containing individual square tubes that function as compartments to keep your material in. I made three tiers to the rack, one with 24 inch long tubes, one with 18 inch long tubes, and one with 12 inch long tubes. There are a total of 10 tubes of each height.
I also added two wheels in the back, along with a handle on the front, to allow the rack to be moved around. This comes in very handy when you need to relocate the rack for cleaning, etc.