I have been using an inexpensive "tool bag" from Harbor Freight, though this is like may other 11" military surplus canvas bags available. It has a rigid bottom and a small snap-closure side pouch. I have a 48V 22ah pack battery in there, plus a little room on top of the battery for a tool pouch I carry. This is mounted to a T-Cycle battery mount - I wanted a quick-release sort of attachment rather than fishing the pack out of the bag when needed, so I used some photo-intended parts I had - an arca-style quick release plate and long clamp. These bolted easily to the T-Cycle battery plate and mount, and the plate is attached to the rigid bottom of the bag. I actually tried a 50-cal plastic ammo box, which fit fairly well, but was oversized and didn't look quite as stealth as the bag.