The key part that you need from Porsche is the mounting bracket that matches the untapped holes which are present on the seat base, you can choose to get bolts and so on from Porsche at the same time, but my suspicion is that they'd be rather cheaper elsewhere. For the sake of completeness, I'll include them.
Porsche parts:
999 141 028 02 x 3 = T25 headed M5 bolts to mount metal bracket to seat frame
996 521 674 00 (LHD) or 996 521 673 00 (RHD) x 1 = metal mounting bracket
Now we're into Mercedes parts:
A0019942845 x 2 These are labelled "part 17" on the Porsche diagram, they're the folded metal clasps that the screws go into which hold the cradle to the mounting bracket
A3019900042 x 2 Countersunk washers
N000000000509 x 2 Screws, which along with the washers secure the cradle to the bracket, screwing into the part 17's.
A0008660214 x 1 This is the cradle itself
Gloria P1DB - this is a specific shape of extinguisher which you'll need, others won't fit the cradle. I bought mine here
https://www.expert-security.de/gloria-autoloescher-p-1-db-mit-kfz-halter.html
I bought everything apart from the actual cradle new, and as I mentioned before as long as cradle, new, from MB is not huge money all the parts including the extinguisher should come in well under £100.
You'll need to tap the three holes on the seat base - but that's fairly straightforward once you've unbolted the seat and tipped it backward.