Has any body just cut up an auto air filter and put in the air filter container? I have seen folks that have put a cone filter on the back of the air box but figured this would work as well as long as you get a similar flow.
I have done precisely that on my CB350F! It worked out fine and doesn't even look that bad...but then, who will ever see it but me when I open the can?
I found a circular auto filter that had the same depth as the CB350F filter cartridge, and then cut a section (removing the wire screen mesh) and mounted it using auto goop glue (which is similar to silicone but stickier and more resistant to gas fumes I believe). Works fine and probably gives about the same filtering capability as an original. You can however find and buy new original filter elements for this bike, but they cost more.