Many people run with their airbox lids off, because the stock airbox lid decreases air flow. However, running with the airbox lid off also shortens filter service intervals, and increases the chance of water entering the airbox. I also really got sick of cleaning the airbox as well as cleaning the stock foam filter every other ride before I went with this setup. Here was my solution…
I purchased a stock airbox lid off of Ebay (usually found very cheaply). Then, I cut all the inside plastic out with a dremel or other cutting tool, just leaving the 1 cm of rim around the outside. This is your “frame” for the filterskin. Then, from Dennis Kirk, or another retailer, purchase a PC Racing Filterskins Part # PC1386, which is the 6″-8″ Diameter and 4″-7″ Length for a Full-Size CR-CRF-YZ-YZF-WR-RM-DRZ-KX-KDX-KLX-KTMs, Suzuki LT500 Quad. The reason is the filterskin is not going over the filter itself like the manufacturer intended. It’s going around the rim of the airbox lid.
Simply oil up the filter skin, install it over the plastic frame you made, and install!
Besides the cheaper price, another benefit is the fact you can just peel a filterskin off if you dirty one up midday, and replace with a new one, instead of having to do a K&N service in the field. However, I’ve never had to change a filterskin mid-day so far.