Re: speaker adaptor rings b5
I've used MDF rings to fit 5.25" units in my B5, but they are quite tricky as the mounting posts are at an angle to the face of the adapter ring.
You also have to use 2 thicknesses of MDF to first clear the mounting posts and form a seal to the speaker hole, and then a second bigger ring to allow clearance for the bigger speaker ensuring it doesn't fout the first ring. The space on the B5 is very tight, not only on the door card but also the depth available in order for the card to fit back on the door (you only have about 60mm including the adapter thickness). Because the speaker is mounted at an angle this actually reduces the depth again.
I managed to install Rainbox SLX components but it was a bit of a squeeze even though they were fairly slim.
I'd try the plastic rings first as at least if they don't fit properly you can use them as a template to make your own.