I am using this drop-in http://www.dealextreme.com/details.dx/sku.11836
I removed the outer spring and it works well with either 2 CR123 batteries or 2 RCR123 batteries. It's dim on one 17670 battery.
I also installed a cheap 2 stage tailcap from one of my old LED flashlights from Dealextreme and I now have a high/low with a R2 Cree.