Originally Posted by ntfday
Another option would be to fabricate one from sheet aluminum.
That's what I would do, just build a an aluminum box from sheet, held together with pop rivets and a hole cut out of the back of it to fit half the depth of the fan blade(s) into. Do you have any friends with a sheet metal brake? If not, a HVAC shop in your town will be able to bend up the pieces for you for cheap.