I used a 20x20x1 taped to a box fan for over a year as an air purifier because I had ferrets, and ferrets leave a particular odor floating about most of the time.
I didn't want to use an odor-removing filter with charcoal on my furnace because they're super thick and put strain on the motor, and I don't care about the box fan.
It worked really well, probably the biggest challenge was securing the filter to the fan: I put my filter on the "out" side of the fan, and it was really hard to keep it attached. Duct tape and the like actually often would fail, and air would just start moving around the filter. For good airflow through the filter I really had to give the air little other choice.
I'm curious if it would've worked just as well had I filtered the air on the "in" side of the box fan... keeping it attached would've been much easier, but I'm not sure if there's any forces that would make "pushing" air through the filter work better than "pulling" air through the filter.
Personally I'd do the filter on the input side of the fan. They're designed to be pretty free flowing to begin with, since they're intended to be on the input side of the furnace blower. It would also keep the fan itself a bit cleaner. Suction is almost sufficient by itself to keep the filter in place.
FWIW, Alton Brown made a jerky dryer using a similar technique, and he used bungee cords to hold the filters to the box fan. That's probably less hassle than duct tape.
I didn't want to use an odor-removing filter with charcoal on my furnace because they're super thick and put strain on the motor, and I don't care about the box fan.
It worked really well, probably the biggest challenge was securing the filter to the fan: I put my filter on the "out" side of the fan, and it was really hard to keep it attached. Duct tape and the like actually often would fail, and air would just start moving around the filter. For good airflow through the filter I really had to give the air little other choice.
I'm curious if it would've worked just as well had I filtered the air on the "in" side of the box fan... keeping it attached would've been much easier, but I'm not sure if there's any forces that would make "pushing" air through the filter work better than "pulling" air through the filter.