I started with commercial air filtration units, but the replacement prices for filter media sent me to the drawing board.
I agree with Alan a quad of MERV 13 furnace filters and a 20" box fan ( plus some duct tape) should do the trick for most of you. Both of "Corsi-Rosenthal box" and "Comparetto Cube" should provide plentiful internet search engine hits.
Shopping the 3M Filtrete line, both the 1900 and 2200 series filters meet or exceed MERV 13 without spending stupid money. At Team Orange Home Store, you are looking for "level 10."
Good luck and best wishes.