Chris Brice built a custom air-intake for his Cosworth Turbo Rush Quadra. Here are his words and pictures of the process:
Couldn't stand the thought of my new 8 inch RS500 cone filter sucking in all that hot air from behind the radiator, so gathered up all my spare ali sheet and a few weeks later....
The first problem was where to take the air in from. I thought about ducting it from the separated flow ahead of the intercooler or radiator but decided against this so as not to affect their efficiency. I'm starting with the intake from below the nose cone, (still not as low as the sump), but I might duct it out to the left-hand side post SVA and before I go near any fords! (I used wing piping to achieve the necessary leading edge for SVA.)
The vertical section of the case is an inch or so forward of the end of the lower case to form a small catchment area which in turn is drained by two 6mm holes. There is also a wire lattice at the front with about half inch spacing to act as a stone guard.
I maintained a cross-sectional area greater than 7 in² throughout, that is to say greater than the 3 inch compressor intake, with the greatest area at the inlet and the whole thing assembled with pop rivets and good old HV-350 and covered with what was left of my turbo blanket.
The air filter case is designed to be accessed from the top following the removal of the intercooler and top hose, but you probably need the removable engine side panels to access all the screws. The casing is attached to the spun aluminium part of the filter with self tappers in the unfiltered outer area, so as not to contaminate upstream of the filter.
Two standard 70mm Samco hoses are used, one 90° bend shorted by 50mm and one 45° bend shortened by 70mm at the compressor end. This still allows the use of the DJ supplied intercooler bonnet duct.
As you can see from the pics you get about 10mm clearance at the waterpump pulley and I'll be honest there is one small dent to give clearance for the front driveshaft CV gaiter!