I am cosidering getting an adapter to put a P60 head on a Surefire E1E. What drop in LED would work well with 1 rechargeable CR123? Later, I may look for an extension to add a 2nd battery.
Gene Malkoff has made some special versions of the M60 to run on a single Cr 123 (I think they run from 1 - 3.6v). They aren't a standard item so you may have to make a special request (and wait!).
There are several threads about this. The one referenced by Marduke above is probably the most comprehensive, so I'll close this one and suggest you continue there.