I think i can make one for about half that price though, I know it wont perform the same but its a nice project for me.
Just need to know if that size reducer would work, do the pipes streech a bit?
Othes have tried to make there own systems and they have ended up paying near enough the same as a GSR kit, which through R&D is proven system.
A half decent air filter alone will cost nearly half the GSR kit and then all the silicone hose lenghts and reducers, ally joins etc all adds up, As an example i just bought 1*22to16mm silcone reducer, 1*16mm ally joiner and 1M of 16mm silcone pipe to fit my oil catch tank breather kit cost with postage £27.15, now consider that you will need a reducer much bigger than that a joiner a 90' bend, a joiner and then straight length plus filter you can see where the costs add up.
Give it a go if you want, I always respect peple trying there own thing, but be prepared for dissapointment if you fail to construct it for half the price of a GSR kit.