Try a web search on "Corsi-Rosenthal box" about making an air cleaner from furnace filters and fans. I have figured it is ok if there are leaks around the edges of the filters, since air circulates through the fan multiple times and gets cleaned further with each pass, but I haven't done any testing with particle counters or anything like that. I do know that the single filter cleaner I made a few years ago helped at that time.