I've got a set of really heavy duty casters I will be using for the construction of my metal materials rack.
I'm going to make an A-frame, 8 feet long, four feet tall, and about 18 inches wide at the bottom, tapering to 6-8 inches wide at the top, with a tray on top running the full length.
The back side will have an upstanding lip at the bottom (1x1 angle iron), and I'll store full-size sheetmetal panels on it, leaning against the A-frame upright.
Inside the A-frame, I'll probably actually have a couple of cross horizontal cross members running side-to-side between the uprights. On these I will be able to slide tubing and bar stock inside the frame.
The tray on the top of the frame will be for cut-offs and drops.
On the front, it will also have an upstanding flange, and I'll either store half-panels or cut panels, and smaller sheet metal pieces, or will store heavier plate (I've got a few heavier panels in the garage, leaning on the wall now).
Wood workers have this same problem--There are a lot of good ideas out there for storing plywood and wood cut-offs: look in magazines like Family Handyman. My idea is based off a similar rig I saw in someone else's hot rod shop, and I modified the idea with that top tray.
I'll more than likely make the whole thing of 1x1 box tubing, and the tray on top will be box tubing with expanded metal grating for the bottom of the tray (so dust and crud will fall through, rather than accumulate).
-Brad