Bah! I thought of this a few months ago! I guess I wasn't the only one. But that was before I started my relationship with my Delta cpu fan, which won't end until I get water (which will be very soon ;D ).
Homemade ones are free, how much do they want for that setup? 37$ with shipping, looks nice though i guess if you trimmed out as much of the ducting as you could it might help airflow through your case. i'm a huge ducting fan but i think i'd use the 37 for something else and make my own.