How about an oil bath air cleaner like my tractor? It sucks air in through a cup of oil and up through oiled stainless steel mesh that is crumbled up like doughnuts. As the tractor runs the mesh gets coated with oil and dirt. When you shut the tractor off overnight the oil drains back into the cup and the dirt settles to the bottom. The next morning the oil is clean. It's a self cleaning filter. Simple. Effective.
Oh, the tractor is 75 years oil and runs perfect. 60psi of oil pressure. I don't know the exact history, but I'm sure its seen more dust in 75 years than all of our CFMotos combined.
Stick with your oiled foam. They are cheap and they last. They may be a pain to clean but they work. Simple. Effective.