I've noticed that quite a few multi-tool holsters come with a little slot that's about the size of a AA battery. Perfect size for a single AAA-battery flashlight. Sheaths like the Wave have two of these. So, what do you carry in yours?
Coast LED "Mini Tac Lite." Comes with a forward click switch, runs on a single AAA battery, inexpensive ($7.00) and works very well. I have two of these (one rides with my Leatherman Surge, the other with my Leatherman Kick).