My DIY proposal:
Once you fabricate and install a simple pair of shields (to keep stuff off the rear of the frame and derailleur) out of a sheet of lexan or clear acrylic, I'm thinking you could just flop two fabric shopping bags over the rack, like panniers. Fasten the bag handles to the opposite sides of the rack with carabiners or bit snaps. Should be fine for toting groceries.
That being said, I did just order a
Rad Saddle Bag (with $10 discount) for little stuff like the bike lock, cable, and tool kit.
[edit - Oops.
I forgot about the possibility of the bags hitting my heels as I pedal. Clearly I have to try it out first, and then proceed with the mods to keep them more to the rear.]