These are the only such cases I've found on the Internet:

http://goinggear.com/aluminum-waterproof-bunker-capsule-pill-fob-black.html
http://www.edcgear.co.uk/ProductDetails.asp?ProductCode=CC049

The first one is sold out and the second one costs $27 US to be delivered to my home. I know one of these cases is not hard to make. However, with the advent of single cell flashlights, one would think I'd have a selection of 100 different variations, each costing no more than a few bucks.

Countycomm in the US has those Delrin containers as well. BatteryJunction has battery cases but they're conventional plastic boxes, not tubes.

I have the Countycomm for both AA and CR123A batteries. They do the job.

A "keychain battery case for single AA battery"?

Don't they call that "a flashlight"?!

I found it at Countycomm.com.
http://www.countycomm.com/batterylocker.html

It's $21 with tax and shipping. I'll make my own.

That's why I alluded to the "Gerber keychain battery locker" above.

http://www.gerbergear.com/Essentials/Lights/Green-Infinity-Flashlight_22-80010

Looks pretty close to the one from CountyComm, except with more, uhm, functionality.

Yep, Gerber got a winner when they bought CMG Equipment and got the Infinity Ultra with it. It was my first regulated LED light and I have a few. Simple, and so far all are still working as designed. Using one of these as a battery case is not a bad idea, although I believe the delrin case is a tad more waterproof than an Infinity with the cap screwed out to ensure no battery drain; dunno, the Infinity is fairly tight, supposedly waterproof to a depth of 10 feet.

You could probably press one of those aluminum key fob containers into service as a battery holder for a couple bucks. Most of them are waterproof and sealed with an O-ring.

haha. I was thinking the same thing, but I'm guessing the case is meant to be waterproof?

Not sure how I like it,but interesting concept and thread.