So this is how I modded my airbox to gain some airflow, and at the same time keep large debris and moisture away from the filter. This may not be the best HP setup out there, but if need be before an all out drag race, the lid can still be removed. Plus it only cost about $10 bucks, compared to the $80-90 you would spend on EHS or Outerwears.
As I've said, I am a diesel technician. This is a cabin filter used in a Mack conventional series truck. Measures about 10x10", and is comprised of a bunch of fibers kinda like the pads on a swamp cooler. It mounts between the HVAC blower and heater core, so air flows pretty easily through the material. For 2 sheets I paid just over $4.
Holes were cut at the bottom of the box (directly below filter) and middle of lid using a die grinder. A dremmel will work too. There is a good bit of hatching on the bottom of the lid that simply needs to be cut off and ground down. Next, I cut the material to fit inside the box and cover, and glued them down with epoxy. (also about $4)
Finally, I ran a bead of clear RTV (had it in my tool box) around the outside edges of the new screens. Doesn't have to be clear, any waterproof silicone will work.
With any luck, this will keep most of the nasty stuff out of the box without sacrificing too much airflow. The material is washable, so after a weekend of riding the box can come off and be cleaned with a low pressure water hose.