This is copied from another thread but also relevant in this one.....
So I just finished my come-alive 3xC Maglite.
Using 2x21700 with electrical tape (I think its 5/8th" wide) around the middle of the battery to the ideal thickness (trial and error in how much tape you put on)
The batteries stay straight and centred and don't rattle, no need to buy anything other than one roll of electrical tape maybe worth a dollar.
The Philips 5761 in FM smooth reflector and using the AW soft start switch.
Works perfectly
I did the same on my D Maglite with extension tube. I am now using 3x26650 again with electrical tape around the middle of each battery, this time using a FM1909, the rest is the same as on my C Mag.
Also works perfectly......
And yes, I purchased a couple of the Nitecore UM4 chargers, easily do the 21700's regarding length.