I would see if small parts storage containers would fit in there, like the popular HF or Stanley ones. Might have to go in sideways. If they didn't, I'm sure you could find the cheaper clear organizers that would fit in there.
Other than that, the only thing I could thing of would be a plastic or metal tray that slides into each slot and lay tools or parts manuals on those.
Edit: On second thought, they might even be kind of neat as a file organizer in a basement or office, as storage for home appliance manuals, bills, receipts, tax papers, etc.