The old furnace-filter-in-front-of-a-box-fan trick is well known, but it has its downsides. If you don’t attach the filter to the box, it’ll fall off when you shut off the fan. Also, air leaks around the filter, making it less effective. You can build your own shroud to fix these problems, but if you don’t have the time you can buy the Filter-A-Fan.
The Filter-A-Fan fits LASKO model #3733 and GALAXY model #4733 fans. Just remove the screws that hold on the guard and use those screws to hold the Filter-A-Fan in place. The Filter-A-Fan shroud runs $35 by itself, or you can buy it with a fan for $60 before shipping. You’ll have to supply your own 16″ x 16″ filter.
To be honest, I’m not really sure who the market of this product is. A business or professional woodworker will just drop the money on a proper air cleaner, and a DIYer who might not want to spend a lot of money is more likely to make his own. Maybe if they lowered the price to $10 or $15 it would be more attractive.