A site called hard racing 101 offers a DIY Ram Air Kit and also prefabricated ram air ducts for the 929 and 954 RR's.
The pre fabbed ducts are new to their site but I have emailed them to see if they still offer the DIY insturctions.
If they do I wanted to see if anyone wanted possibly split the cost of getting the instructions??
They say they achieve gains even on a static dyno without a PC III