Hmmm. After trying a few things, I think the muffler idea can work if you had a high-CFM fan blowing out... My current slot blower/duct is so strong that all available openings in my computer case besides is sucking air into the case. Even my PSU fan's apparently helpless against it -- the amount of air blown out from it seems drastically reduced.
Since it's a slot fan I'm trying to muffle, I can't use the product on that website anyway. So, I'm going to attempt to make a L-shaped thing the width/height of the slot blower, with the inside lined with acoustic foam, which should absorb some of the sound.
\_____ <- fan here
I'll post the results when I'm done.