I recently acquired a pre-owned early-serial Surefire U2 that could accept 18650 cells. To increase my options for cell-usage, I decided to put regular 123s in it, but there was battery rattle as the diameter of the tube was much larger. So far, I've only read about people asking for the inserts to be removed, but never the other way around - correct me if I'm wrong. So on to the hunt for an insert that would fit nicely into the tube. Various items were looked at - the kids toys, candy containers, glue sticks etc etc. Of all things, I found an aluminum tubing that fit the batteries perfectly AND could go into the tube with minimum effort. An alu tube within another alu tube - perfect!

A 60mm section of the tube is cut, then both edges filed flat and smooth. I had to sandpaper the inside of the U2 a little as the boring was not very smooth and the tube wouldn't go in easily. A little more sanding on the outside of the alu tube and it now fits into the U2. I can remove it by inserting a finger into the tube and twist it out.

Hopes this will be useful for those who've either lost their inserts or who'd like to have the option to run 123 cells in earlier U2s.

The 2 tubes

Not sure if you can see clearly in this pic, but there's a lip at the end of the U2 tube that'll allow only 123 sized cells to enter. 17670 and 18650 cells will have to be loaded from the front.

Tube part-way in

Tube now fully inserted

Tube fully inserted - front view