The original bottle set has soft straps securing the bottle (see last picture). While you can easily grab the bottle and have a life saving sip of water at a redlight or Autobahn Stau stop-go traffic, it's nigh impossible to put the bottle back with one hand. These retainers replace the straps, allowing easy slide-out/slide-back while still securing the bottle well during travel. For a couple of years I've had large diameter hose clamps doing this job and while that worked perfectly fine, it was butt ugly (and I'm not even a café poser but a hardcore touring addict).

I wouldn't recommend PLA for this. I printed mine in PETG for now. ABS/ASA might be even better, but OTOH the bottle I typically use in this is also in PET.

This version is for the original TRAX aluminum 0.6 liter ø73.5 mm bottle. See also my alternative model with a slightly different diameter, suitable for ø71 mm 0.65 cl Imsdal bottles. Other diameters are easy to add, just ask me! Please note that you can't merely scale it: The ends are designed very carefully so they can barely be fitted, and then can't unmount with a bottle inside. Actually I'd be very surprised if it unmounts even without the bottle securing it.