I've seen where some stuck a foam filter element between the frame and airbox (not to easy to service there for sure).
I've also seen where they used a metal mesh similar to the T-O airbox internally to hold the filter in place.
The ETI and airbox situation has been the main reason my bike has been in pieces for 2 years.
No matter which way I turn I get stymied one way or the other.
Frankly, I just want to ride the thing, so I'm temporarily throwing in the towel on the airbox and just reinstalling the OEM one for now.
I still lose 7.5 lbs with my ETI tank, and another 7 lbs with the Shorai battery, so I can accept that.
In the mean time, I'll try and start learning some fiberglass or carbon-fiber fabrication techniques.
My ultimate goal is to combine design elements of the T-O airbox with a closed top design as the OEM or Moriwaki units use.
I think the T-O airbox is great design for racing, but it offers too many sealing surfaces to potentially leak for every day use, and it doesn't work with ETI fuel tanks.