they sound fucking amazing dude.
as for the spacers, you wanted a spacer so the sub doesnt bounce off the bottom of the enclosure, and of course the top spacer is so it doesnt bounce off the grill.
I didnt have to file the fins either, as i didnt have any fins in my 2011 e90.
as for the top spacer, im not sure. 16mm is fucking tiny. i used 1/2" acrylic for my first try and it wasnt tall enough. However i have plenty of room between that and the seat rails. So i need to use 3/4" next time. For now the carpet is folded in and out of the way and left them open.
I cant chime in on e92 vs e90 seats though.