For the record, I have:
-Used a milk crate, but stopped due to my backpack being too large to fit inside.
-Strapped the pack to the platform, but messing with the packs straps all the time got to be tedious.
-Used a flat beverage crate. Most useful solution by far, but blocked my rear lights too much.
-Had some saddlebag type panniers from Knog that I had heel strike with, so traded for the mess bag in question (why I started this thread).
-Used some old Nashbar Townies that I had better luck at avoiding heel strike, but a full backpack wouldn't fit, they don't shed water all that well, and when I did use them both for grocery runs, the load made the bike respond sluggishly.

I've decided that though this bike does have the proper eyelets for mounting a rack, it's happier when it doesn't have one. I may yet decide to put a rack back on this bike, but it won't be the current one. Until such time, I'll roll without.