I picked up a compact plastic toolbox to house my 36V 20AH LiFePO4 battery. I couldn't see putting such a fragile component in a rear rack either by itself or in a canvas/fabric rear rack bag so I went looking for a hard case that would fit on a rear bike rack. I found this very inexpensive toolbox at Home Depot and it fits the battery perfectly! There are even spaces for the cables that come out of the battery so that they don't have to make a hard bend to reach the BMS.
I've ordered the rear rack from REI and it should be arriving tomorrow at the local REI in Rockville.