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

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.