Tracy, those oil bath air filters can work very very well but are also somewhat restrictive but probably less so than the focus unit. Nothing like doing an oil change for the air filter! Lifetime cost is next to nothing and they last literally forever. All the oil in the airstream however may affect the MAF sensor though.
A very large paper element seems like the best solution. Low pressure drop across the element and very fine filtration. Gee, maybe retrofitting a larger replaceable airbox/filter from a larger engined Ford truck would do it?