Donaldson make pre cleaners to go on the top of the snorkel, they are big clear cans often fitted to tractors. I don't think it should be used as a substitute for a proper snorkel however.
Some people just stick a k and n cone filter straight on the top of their snorkel and run it straight to the turbo. My mate has done it to his 90 but it doesn't have any protection from rain, snow and mud so could potentially damage the engine in the long run.
I managed to get a K and N cone type filter inside the mushroom top of my snorkel with a big of cutting and bodging. It seemed to work fine and the 300 drove well but now I am rebuilding the 90 I'm planning on reverting back to a proper air can on the basis that despite it being more hassle, it's not worth buggering a good engine to save a couple of hours work.
