In my old thermos the cap's sealer broke into three pieces:

So I reverse engineered the cap's sealer in FreeCad.

The initial MK1 model has been designed exactly as the original sealer, but it was not rigid enough at the bottom part, so the MK2 has been made.

Tested with PLA PRO, but still after using boiling water the cap's sealer deform.

Printing
Print with bottom on build plate standing.
Filament - ABS
Infill - 80%
Layer Height - .15mm to .20mm
Supports - Needed for MK1
Perimeters - None
Additional Parts - None
Bed Adhesion - You may want some glue stick to ensure good bed adhesion, but in general it's not needed.
Rafts - None
Scaling - None
Filament Change - None
Post Printing Notes: May need some grinding.
Tested with hot hot tea, and unfortunately the cap's sealer went pliable.
The author marked this model as their own original creation.