I returned some equipment to REI to find that they are having a sale this weekend. I picked up a new Topeak MTX rear pack that fits on my Topeak Explorer MTX Disc Mount compatible rack. There's a slide mount for the pack that makes installing and removing the pack a snap.
Also, my new Bosch Fat Pack 36V batteries (all 6) fit in the pack absolutely perfectly. It's as if they were made for each other!