For you Do-it-Yourselfers!
I found an awesome 12 volt 5ah battery holder!.

It is made of neoprene, same stuff they make wetsuits out of, hopefully this gives it a little water resistence. At the very least, it provides a pretty cushy feeling, which might convey some impact resistence/padding.

It has a sewn in belt loop on the back, so you could ran a strap through it to attach to a rack. It also has 2 sets of quick release clips. One set was made to attach a shoulder strap (perfect for running a strap around a seatpost); the other set is on the back and was made to attach to luggage or to use a strap as a belt. Just using the one set of quick release clips (without an added strap) on the back it clips perfectly to my rear RANS rack without needing anything else!

My 5ah 21 volt SLA battery fits snug inside with room at one end... perfect for a little wire and my fuse. It has an added small front pocket that can fit extra fuses and a mini-mag light (i.e. emergency backup bike light).
(this msg also added to the "Total Geekiness" thread)