I had a battery blow on me once in a bag. It was a "WTF" moment as I checked the bag and found out the battery (alkaline at the time) ended up with the coin change and shorted and blew up. Thus why I've been using as thin a battery case holder I can find ever since. A more strip down version for space could be to gun tape the batteries together then gun tape the tops but on hot days and after a few hot days and the lights drained and you forgot your batteries and you reach for you backup batteries in the pouch and you get a nice sticky goo coating. Icky. I'm going to see if a $5 bill can fit into the battery case for emergencies or if not then tape two transit fare tokens inside so they don't contact the cells and also I don't worry if the tokens get smiley as they're being used for the bus ride then if somethign mechanical fails on the bike.

Jarery, thanks for the helpful feedback on the Arkel seatbag, I think I'm going to give it a shot. The quality looks far better than anything else I've ever seen before.