Hey man, don't despair, there is a cheap solution. Build a box for your squirrel cage. Simple as that. There are plans on the net if you look. Basically, you want to mount the output of the fan flush with one side of the box, and then seal the rest of the box except for one intake hole. Caulk the shit out of the inside of the box, and mount your filter on the intake hole. Ta-da, problem solved. Squirell cages aren't air tight, so unless you're pushing through the filter, you gotta box them hoes up first.