My go to brand for odd sizes, high quality, and a great belt clip is
Ripoffs. Due to the odd dimensions of the UB3T's head, the only holster I found that may work well is the
CO-159. Although the UB3T is a tad longer than the sheath, from experience, I would bet that the adjustability of the velcro flap would accommodate the extra length of the light.
The challenge is accommodating the heatsinking section below the bezel, which Surefire states is ~1 5/8" in diameter, which the 159 looks like it will accommodate. With the extra room available in the holster below the heatsink, you could possibly fashion a way to store extra cells.
Or, if you just want to fill the extra space, you could snugly fit a short section of PVC pipe into the bottom of the holster that is shorter than the grip ring, and has an I.D. greater than the 1" diameter of the tailcap. That set-up would allow the UB3T's tail end to slide in and out easily. Just a thought.