I mounted a marine (white) fire extinguisher under the front of the passenger seat. While NOT AUTHENTIC BMW it will serve the purpose, (besides you can hardly see it)
With the sport seats, run the seat all the way back, you'll feel a raised area, (on my 06 3.0) you can drill into the sheet metal thru the carpet and just use some fat 1" sheetmetal screws in the metal bracket that comes with the marine extinguisher...
ps, it clears the seat when you run it back forward !

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BMWCCA # 12660
14 GLK 350 M/B (hatefull POS gone)
05 CLK 500 AMG Cabrio
07 530xiT
09 Boxster