Oh and I noticed looking at the 20" hurricane box fan that the cord comes out of the back grill, and might be in the way of the filter. Might not be a big of a deal.
Also, Lasko sells a box fan now designed just for adding furnace filters. I like making them DIY though, and can make 2 for the price of one of these.
They also make little universal kits that you can add to most the box fans, by taping the or clipping a extra hardware on that holds the filters in place..