Had a similar issue and found a “Geruchsverschlusseinsatz” what helped - roughly translates to “odour trap insert”. For the diameter and depth in my flat back then this was fitting: https://www.dallmer.de/de/produkte/482013_Geruchsverschlusseinsatz-zu-TistoPlan.php#anchor_897ac650_Accordion-Datenblaetter-und-Ausschreibungstext

It’s in German but when you scroll down you can switch to English (but at least on my mobile I only get to the Home page then and not to the product anymore)