Mizu S6 Bottle Cap Verschluss Thermoflasche

My cap broke into two pieces and just replacing it with a new one wouldn't solve the design issue the cap has.
As the metal head is just glued to the sealing black plastic part, it's just a matter of time the new cap will break again.
The thing I created fits just tight into the black sealing part of the cap. I glued it but it fitted even without glue.