Recently I got the mounting L bracket from BMW - it cost around US$40 and is mounted under the front passenger seat. It's held in place with six bolts and the holes are in place under the seat.
Any aftermarket fire estinguisher with a steel bracket can be installed using this kit.
Installation is easy as mentioned 20 minutes tops. Undo the four torx screws holding down the seat and recline it against the rear seat and this way you don't need to unplug the airbag loom and disconnect the battery.
Install is neat and tidy and because it's fitted to the seat it will move up and down when the seat is moved.
I will post the part number and a couple of pics from the install later.
E92 M3 2007 - ///PE with Test Pipes and BPM Stage II; CF Interior & Splitters; CIC with Connected Drive