You've probably at one time or another seen
a version of a box fan air filter. But recently while shopping at the local
home center, I came across some vinyl siding J-channel. (Its normal use is for
trimming around doors and windows.) I found it could be used for mounting
furnace filters to box fans much easier.
The advantage to this design is
that it firmly holds a 20" x 20" furnace filter, yet it allows for easy and
quick removal of the filter for cleaning or replacement.
Start by removing the plastic grill.Then attach the channel to the grill, using
some narrow wood backing strips to screw into. I did this to three edges of the
inlet side of the fan, like you see in the top photo. Finally, replace the grill
back on the fan.
Standard vinyl J-channel is
at most home centers in the siding section.