I understand this is an old thread, but I found it searching for answers so adding to it:
I've been searching around the net many hours trying to find someone who makes a round 45 mm diameter battery holder for three 21700's, especially a holder which can allow me to string two or three of them in series for 3 X 3 batteries. Not finding anything but I did reach out to an individual who has a 3D printer who might a be willing to make a carrier like that for me, but nothing back from him yet. He responded he would ask his designer if this can be done for me.
For my application, none of the cells would be electrically in parallel, I would use the three strings as distinct or separate voltage sources with the negative ends of all three strings tied together, just the positives separate. Why separate strings of cells? I don't want to parallel two Li-ion batteries which can be charged separately because if by accident I paralleled a fully charged battery with a partially discharged battery, that could be a very big POOF. Too much current could flow between them if both are not the same voltage, or very close to it. I wouldn't want to parallel two batteries which were even .1, or 1/10th of a volt different. Maybe that wouldn't cause a problem if two were that close in voltage from one another, but I don't want to find out for myself at the risk of an explosion, ruined batteries or physical harm to myself etc.
There is a possibility of using a plastic honeycomb battery holder for 3 X 21700's, stringing as many as I want and then wiring them together as needed, I ordered some from ebay but have not received them yet. I don't know if these can be cut to allow a tri-cell configuration which has an OD of 45 mm or less, but ordered 10 strips to find out:
https://www.ebay.com/itm/133483378717 - 10 each strips is the smallest order I could make.
I ordered some Nitecore NL2150HPR 21700 cells which can supply 15 amps continuous, with those I could have three separate 8.4 volts sources (when fully charged) for a capacity of 5000 mAH per string, total 15 amp hours, although split between several laser diodes.
