I ended up with a engwe rear trunk bag. Simar to topeak, rockbros etc. Wasn't sure about it at 1st, but then I started to warm up to it. Now I think it's great. Initially I wanted a bag of some sort to stow my gear, but more importantly a spare battery. At first I didn't see how this all bag could do this. I had a bunch of nice packaging foam from the box my bike came in, and I used that to line the bag so the 13ah battery would fit snug. Granted the last 4 inches of the battery sticks out the back, but it's secure. I plan on testing it today. I also tried a pair of Roswheel canvas panniers, but I think this bag does the trick. I also have tools, pump, etc etc etc. Another good point for this trunk bag is that it doesn't get in the way, and it has hidden panniers, straps, rain cover, and the bag expands a bit. I'm happy with it.