Would you like to install different speakers and need some recommendations?
In the Golf 4, I personally like the Hertz HSK 165 very much. The Peerless Definition 6 or various Exakt! models are also at least just as suitable! Of course, it's best to listen to them yourself! Frank Miketta in Celle (www.carhifistore.de) is a very good dealer, but where are you actually from?
By the way, there aren't only adapter rings made of plastic, but also those made of metal.
The more securely a speaker is mounted, the more powerful and dynamic it can sound.
You should also use some damping panels to reduce the resonance of the doors, which will minimize the rattling and booming sounds.
My mid-bass drivers are mounted on steel rings, which are then bonded to the plastic adapter rings and the door using filler.
A tighter fit would only be possible by using red brass rings or by welding.
Best regards,
Daniel.