Here's an under-shelf storage idea that a customer sent in. She has hung milk crates from hooks screwed into the shelf 2x4s. She indicated she uses her crates to organize and contain small items, and still have them accessible.
We have recreated her idea in our "studio garage" and produced the picture above. (We swiped one of our grandson's ball collection to fill ours up.) The milk crates we used are heavy duty plastic and are 17" long x 14" wide x 11" high. Crates this size or plus/minus an inch or two, are widely available in big box or container stores, and many places on-line.
Shipping's kind of tough for on-line ordering, however, since we couldn't find any of the plastic one that would "nest". If I did have to have it shipped in, I'd probably try to get some metal one with sloped side that would ship more efficiently. If you get the 14" wide size, you'll need three crates and six screw-in hooks for each 48" shelf section.
The cheapest we found were at Lowe's Building Supply for $5.98, in stock for pick up. Check your local store. [Note: Price increased to $5.98, 11/2016.] Drop us a line if you have any suggested improvements.