For a pair of 123's, try an APS film can (your local minilab will probably give you a handful of those for free). They're elliptical shape and hold a pair of 123's side by side just perfectly. Tape it shut so the lid doesn't pop off.

You could also think about the Surefire Spares Carrier (I don't remember exactly which one). It holds 4 (or maybe 6) CR123's and a spare lamp assembly for the G2/6P series.