I was in Walmart the other day and picked up what was meant to be a handle bar bag for $10. Mounted on the back of my Brooks B72 it seems to serve well.
I had to trim the velcro straps to go through the holes in the B72, and fasten them kind of crosswise. I had it out for a five mile trip today with a few tools, a patch kit, and a spare tube. It seems to be secure. It could use some kind of strap around the seat post to keep it from boucing, but other than that it seems to work OK. It looks right at home on the elderly Dunelt.