Well, I found a couple sources for Fischer CBox storage modules (the OEM supplier for the cassette drawers) that would allow conversion of the cassette holder into a 4 drawer CD holder:
But what I really wanted was a storage area for miscellaneous items. Here's what I decided to do. I removed the cassette boxes and I'm going to bend up a cubby box out of black anodized sheet aluminum. There's a lot of depth back there -- I'm going to make it 8 inches deep. I'll line it with black adhesive-backed felt and I'll attach it to the top of the radio shelf with double-sided foam tape. The only downside? No lid.
Will post pictures when it's done if anyone's interested.