I think the most versatile set up would be the Solarforce L2M. The body of the light can break down to a 1 CR123 size as well as a 2 CR123 / 18650 size. That gives options later if needed. http://www.lighthound.com/Solarforce-L2M-Host-for-1x123-or-16340-Black-Type-2-Anodized_p_2982.html
You would only be able to take advantage of the battery options if the dropin supports it. Notice the voltage range of this dropin http://customlites.com/P60-Cree-XPE2-Colored-Emitters-P60-Color-XPE.htm 2.8 -6 volts so 1 primary CR123 operates from 2.9- 3 Volts. It would work. So would 2 CR123. One lithium ion battey wold work too, 18650 for the longe configuraton or 1 RCR123 for the shorter configuration. Lithium ion operates at 2.9- 4.2 volts.
You would only be able to take advantage of the battery options if the dropin supports it. Notice the voltage range of this dropin http://customlites.com/P60-Cree-XPE2-Colored-Emitters-P60-Color-XPE.htm 2.8 -6 volts so 1 primary CR123 operates from 2.9- 3 Volts. It would work. So would 2 CR123. One lithium ion battey wold work too, 18650 for the longe configuraton or 1 RCR123 for the shorter configuration. Lithium ion operates at 2.9- 4.2 volts.