Discussion of LED choices and interaction with reflector fine-tuning. Important to get optimum results.
Double o-rings not needed, a single one seals well, done properly. This is not a dive torch.
Retaining ring in head should remain clockwise. The tube is rarely removed from the head, and when tightened up also tightens the ring, if somehow is a little loose.
It is the tailcap that needs a left-handed ring, and/or redesign so the tube bottoms on the metal of the cap, rather than the ring. This is the piece that is unscrewed every time the cell is changed, and tends to loosen. Un-anodised Al to Al contact works here, the constant use keeps the surfaces clean.
Better thread engagement between tube and head, for mechanical and thermal reasons. Square coarser thread not actually necessary here, it should rarely be touched. In fact undesirable, the head should be on tighter than the tail, so if you grasp the head rather than the tube whilst unscrewing the tail, it is the tail that loosens.
Consider masking off anodising from these threads, and inside the head where the driver rests, for an additional current path.
20mm driver is good. If Lexel’s is good I won’t want to change it. It was the original poor one that had to go, and at that time the obvious swap was to a 17mm one, at best a marginal fit, in my case I chose to use a 20mm adapter and file things down to fit.
Short (18350, 16340 etc.) tube option.
Its sounding good.
Hope all goes well for your scan.