trikbldr wrote: <..."why not take some chicken-wire and make up your own foam filters? Simple construction and you can fit it to the stock box".>
Last year I took a stock air-filter frame and used it to mount a felt filter medium. There was some considerable work required to remove the paper-element. The top, bottom, and ends of the stock-filter paper-element are "glued" heavily. The paper element is easily torn out but removing the adhesive was difficult. I ended up using a propane plumbers torch to heat the glue and the metal to a point where I could scrape the softened glue. The fumes are not all that pleasant ! I used a "Kawasaki" OEM filter. After market filters may use a more easily removed adhesive. I started out with 3 layers of felt but eventually reduced that to 1 layer. The retaining rod was originally positioned to hold those 3 layers so a piece of self-stick weather stripping stuck to the rod, takes up the space and holds the single layer. If I do go to an "air-filter' foam, the supplemental piece of foam can be removed. The height of the felt is cut a bit long so it is in full contact with the frame ends.
A pleated filter gives more surface area which arguably would give a longer service life, but I don't run a filter 'til it's black. I might replace the felt with "Air Filter Foam" if I come across some but for now I'm satified with the felt. It seems to be effective, easily cut and mounted, and so cheap I throw it out and put in a new piece. Now that my GM coil-pac has been relocated to above the cam-cover, the tool-tray is back to being useful but more significantly it's back to being easily removed to access the air filter. I was cleaning the stock air filter a couple of times a year anyway so changing the felt filter a couple of more times a season (if required ) and with easy access is of no concern. I'm more of a "proactive/preventive" maintenance type.
On a personal note: " I would NEVER" hack my air-filter box to accomodate an ill-fitting/badly-designed filter - K&N or otherwise! I hacked the original years ago in some fetish attempt to re-engineer the air filter system and ended up buying 3 new air-box pieces to get back to original filters.
Hind sight is 20/20 !
What seems to be ommitted in the few comments from those who have or are thinking about "enlarging" for a K&N filter: Enlarging the opening at the "top" of the air-box is easy. But if you want to "Split Hairs" - The bottom requires enlarging too because the air-filter's foam seal sits on a lip created by the "snorkel" part of the assembly! A previously used filter will have compressed gaskets and after some time may not creat a proper seal. Having said that: As long as the filter's foam seal is in full contact with the bottom of the tool-tray (lid) as well as sealing on the bottom: should be OK.
Certainly something to think about.