These pics with extinguisher removed. You can see the plate behind the extinguisher holder how it's bolted to the under side of the seat. I had to move the anchor point on the extinguisher bracket by an inch or so to line up with the hole on the OEM bracket.
When not in use one can remove the holder by undoing the two bolts - provided as part of the OEM kit.
I know a few members are the BMW Club got custom brackets made costing as much as US$250. This is the cheapest option and including the extinguisher it all cost only US$65.
E92 M3 2007 - ///PE with Test Pipes and BPM Stage II; CF Interior & Splitters; CIC with Connected Drive