Remixed Devin Montes' design (full credit to him) flipping the cap over so the inside of the cap is printed flat rather than bridging so it is smoother and less likely to collect yucky stuff. (The Ender 3 bridges quite well, but flat printing is still smoother. This should help you if your 3D printer, settings and material combination doesn't bridge well.) After doing that I realized the strap needed to be longer, so this is actually the second re-mix.
The pictured one is printed in natural clear PETG with a very fine layer height. The strap was softened carefully just a little bit with steam to set in a curve. It is very smooth and I'm pretty pleased with the result.
Devin's original is: https://www.myminifactory.com/object/3d-print-camelbak-mouthpiece-cover-74558