I created and tested with my tank a replacement rubber seal. I printed it with soft TPE filament and the result is great – smooth turning without leaks. The model itself is rather simple: height 5.0 mm, outer diameter 20.0 mm, and inner diameter 11.3 mm.
Recommendations for printing are as follows:
the main secret is filament – it should be really soft. I tried several, and Recreus FilaFlex82A worked the best. I suggest downloading the following printing profile for that filament
layer thickness: 0.2 mm
infill: 5-10%, Gyroid
two top and two bottom layers
speed: 90% or slower
TPE has enormous adhesion, make a test print if you try it for the first time. Reduce adhesion with a glue stick, if needed.