I don't recall it being any challenge to install. Question (and maybe my memory has a whole in it) - but is there a ringed flange on the base plate? I don't know why but I have some memory of removing one on my car when I installed it Paul. I think that may be your problemo buddy...
Yep... Pretty sure that's the issue senor!
Scroll down to the section for 911 Turbo / 930 / C2 Turbo Intake:
FABSPEED SUPPORT & INSTALLATION INSTRUCTIONS
It says:
Remove factory Porsche steel air box assembly.
Pry up round steel ring with rubber insert with flat blade screwdriver that was at the base of the CIS plenum. You have to remove this ring as this is where the airfilter attaches with a hose clamp. Install new BMC F1 air filter and tighten hose clamp.
P.S. - Text me next time man! I could have answered your question in 5 seconds! Duh!
Chris