Some time ago --after doing some research -- I discovered that, with only slight adaptation, a K&N air filter HA-6592 works great in the stock z600 air cleaner box. The main difference is that the dimension between the seal and the metal plate is less than that of the stock filter element. (I want to say "height" here when describing the dimension, but since the filter sits on its side, it might be more accurate to say "depth" rather than "height" for this application.) I took care of this dimensional difference by not using the stock air filter element retainer clip and instead rigging up a custom part to compress the filter element's seal against the front inner surface of the air cleaner box. (The custom part is made using a round head 1/4-20 machine screw [1-1/4 inches long, as I recall], two 1/4-20 machine nuts, a fender washer, and a 1/4-20 coupling nut. When I get a moment, I'll post a pic or two of what I came up with.)
Since it's a K&N filter, it's essentially a permanent filter element that you apply oil to and then periodically clean and re-oil. You can get the filter on Amazon for $41.48, but you also need to by air filter oil, cleaning solution, and sealing grease (if you want to achieve a really good seal). But it's the last filter you should ever need to get. And it's a K&N filter, so the filtration is great, with minimal drag on intake air flow. And this arrangement allows you to use the stock air cleaner assembly, complete with the intake pre-heater function for cold-weather driving.
BTW, this K&N filter was originally intended as a replacement air filter element for a Honda XR650L motorcycle, so there's no concern about whether the filter should work with the z600 engine displacement.
Richard