Take a heat gun and heat up the plastic duct. Make some relief cuts in it first.
Shove it in there while its hot or take a dowel or something and hold it there until cool. Then, take some dynamat or something and seal the duct up. Voila!
I see what you guys are talking about now. Moved my deck up from the POS bottom spot but it sticks out just a bit (frame is flush but faceplate sticks out). I just NOW finally received my heat gun I bought off eBay (Milwaukee variable 90-1050 degrees for $52 SHIPPED) so I'll pull it all back out and heat it up to get more space.
Question: Are you guys losing the amount of air blowing out the center vents by doing this?