Making the 911 that Porsche were too sensible to consider

Where did you get the M509 kit from?

There's a far less visually pleasing version on eBay:
https://www.ebay.co.uk/itm/KAP-INDUSTRIE­S-FIRE-EXTINGUISHER-BRACKET-FOR-PORSCHE-­911-WITH-EXTINGUISHER/173795780356

You can order the majority of it from Porsche for very little money if you have the part numbers, however they have iterated through a few designs of extinguisher bracket as the extinguishers themselves have evolved - the bracket itself is a Mercedes part, and again is cheap ordered direct from Mercedes.