MDF Front Rear 100mm 4" Speaker Adaptors Rings for BMW 3 Series F30 F31 Z4 E85

BMW F31 (2011-2019). They are an excellent alternative to plastic adapters, which are often unstable and which never allow speakers to play to their full potential. Because mounting surfaces that are as solid as possible are one of the key reasons for clean sound, even at high levels.

eBay