For a Boxster, this is a 2-stage process. You need to get an "extinguisher mounting bracket", which mounts under the passenger seat. I called Black Forest and had them get the bracket and install it in my Boxster last March. By the way, there is one on eBay now for $69.99 - google "boxster extinguisher bracket" to find the link. Then, the metal mounting straps for the extinguisher have to be fastened to the bracket - this is different for each type of extinguisher mount, and usually involves drilling a couple of holes in the under-seat bracket to match the extinguisher holder. You can buy the extinguisher directly or just call one of the excellent Porsche shops around town and have them buy it and install it.
As noted before - just make sure the extinguisher has a strong metal strap securing it to the mount, rather than the cheap plastic ones used for home kitchen extinguishers (some home kitchen extinguishers are actually the right type of extinguisher, but the inexpensive wall mounts won't cut it).
I think everyone buys the Brey-Krause under-seat mounting brackets for Boxsters and Caymans. The mounts vary by model and year
http://www.bkauto.com/porsche/fire_extinguisher_mounts_seat_rail_porsche.php.
Ease of install is all relative to your skills, patience, and available time... I don't believe it is particularly difficult, but you need to remove the seat. Brey-Krause has all the instruction sheets on-line, so you can walk through it and see if it is something that you want to try (see
http://www.bkauto.com/porsche/instructions/R2040.pdf for instructions on a typical B-K bracket mount).