Just ordered from extremeairvents.com, 200 US dollars shipping included for an xp kit, I almost went with the slp stuff but without a metal mesh or heavier screen in conjunction with their pre-filter material they look a bit too fragile. If you contact extremeairvents.com make sure you inquire about the upper vent, which isn't pictured on their site, I think it makes more sense than the knee vents people are making, which I think your knee is sure to go through landing drops and donuting or downhill turning etc.
nate