TrickBlaster'03 asked me for some info regarding this...it came from my old web page. Kinda cutting and pasting here.
I like the airbox because of the water crossing, mud and heavy dust we run through in the woods.. So, here goes.
If you'll look at the top of your stock airbox you can tell by the picture what I did. A spiral cutting bit works great to punch thru the plastic and make a smooth cut.