I have return duct 20x25 with 1" opening at the top to insert/remove standard filter.
Was thinking about upgrading to 5" media filter.
a) Install AirBear/Honeywell/Carrier/TotalLine cabinet
b) Modify duct, cut additional 4" opening at the top, rivet 4L aluminum brackets for the filter to slide in/out, fab simple cover like.
I lean toward the first choice, because there you increase chances of error with field fabrication.
Just adding a couple of rails does not guarantee that the air will actually pass through the filter.
I like option b ,if that means you can use a 20X25 media filter,and you put some gaskets on the down stream rails to prevent bypass air.
What size and cfm system?