It is also possible to start with a SureFire 6P, and add an A19 extender (to make it a 'virtual' 9P), or just start out with a 9P. That makes it the right length for 2xAA. Then you add a SF A14. That is the extender to turn the 6P into a 6R rechargeable (for using the SF B65 nicad battery stick). When you add the A14 to the 3xCR123a length, it makes it the right length for 3xAA. The A14 isn't EXACTLY the same length as an AA cell (it's just a smidge longer), but you can play around with it and make it work.
I am using a similar setup for a dog-walking light (although I've got a backup light with me). I just recently decided to finally get into some rechargeables, and I'm starting with NiMH AA Sanyo Eneloops. Since I'm not doing rechargeable lithiums yet, I'm trying to be creative with the Eneloops, and I want to acquaint myself more with incandescents, since I have plenty of led lights. I've got a P60 in a SF 6P + A19 + A14 + A14 running off of 4x'1.2 volt' Eneloops. It's a bit long, and the connectivity is a little bit funky, and I've got 2 o-rings that I dropped into the regular 6P bezel against the lens, to force the P60 back towards the battery contacts as much as I can. Since this isn't 'mission-critical' usage, it seems to work okay. You may need to play with the tailcap 'setback' some. Being new at this, I actually started out with 5 Eneloops end-to-end, and blew the '6 volt' P60, since I assumed 1.2 meant 1.2, when it turns out you can actually charge Eneloops up to about 1.4 volts, and since the P60 should be thought of more as being 5 volts, since the 2xCR123a voltage drops quickly 'under load'. Somebody correct me if I am using the technical terminology incorrectly.
I also have a Malkoff M60LL in a G3 FYL body, running off of 2xAA lithium L91 primaries (for 18 hours of regulated runtime), and another SF lego with an M2 bezel and tailcap, and a 6P body + A19 + A14 on 3xAA lithium L91 primaries, to run a Malkoff M60WLL for 'only' 12 regulated hours, but at a brighter output. The surprisingly versatile performance characteristics of the M60LL driver has been discussed in various threads on CPF.
The AA cells do rattle around some. In the SFs that I am using primary cells in, I went to the hardware store and got some black vinyl tubing. I can't remember the size. It fits in the internal diameter of the 6P no problem, but is a tight fit over the AAs, so I just cut a very thin 'donut' section that I can just barely fit on each end of each battery.
If you are going to start using rechargeable AA cells, just be sure to keep in mind the difference between the total voltage of the rechargeables vs. the total voltage of the primaries. A lego setup that is pushing the limits with the number of rechargeables being used, might be enough to blow something if you take out the rechargeables, and then just drop in primaries without taking an extender off.