I bought my Zilla w/ a k&n filter set up for the stock air filter arrangement with the clip. It leaked and let sand past into the engine. I'd recommend the clamp on filter flange made by Shawn/sblt500r, it allows you to run a trx250r k&n filter ha-4250. Much better set up, seals better.

I have a k&n air filter for both setups But he's right, the stock box will allow dirt to get in between the airbox and the filter.  Best to get an elimination kit.   Let me know if you need the trx250r filter.  Just sitting on a shelve at my house.