HVAC baffle box design

I am currently finishing the last few tasks needed before I can drywall the basement. One of these is solving the problem of minimizing the sound that will escape the soundproof rooms through HVAC vents (both return and supply). This is proving to be one of the most technical and crucial elements of the soundproof rooms. They are airtight rooms, and so there are serious considerations to be made such as making sure there is enough oxygen supplied to the room!

Here are the steps I have taken so far:

1. Research: I have mainly read up on studio construction forums, and how other fellow DIYers have attacked the problem. Some of the best threads are: http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=10303&start=0 http://www.johnlsayers.com/phpBB2/download/file.php?id=20076 http://www.avsforum.com/avs-vb/showthread.php?p=21117461&highlight=hvac+vent+sound#post21117461 http://www.gearslutz.com/board/studio-building-acoustics/526036-lots-duct-questions.html http://www.gearslutz.com/board/studio-building-acoustics/543058-hvac-baffle-box-construction-plans.html http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=1929&hilit=silencer&start=74 
2. Conclusion of research: The main principle behind the baffle box design, is that the 2D area of the air flow path through the box should be twice the size of the duct you are connecting to. This is to allow a sufficient of volume of air to move at a slower pace through the baffle obstructions.
3. Assess the room available to place baffle boxes in the basement. Measure the external dimensions for the baffle box in each location (in my case there are 3 supply vents and 2 return vents).
4. Design: Google Sketchup appears to be the perfect tool to design these baffle boxes, to a) ensure the design meets HVAC requirements and b) make construction of the boxes easier. I have been playing around with Sketchup, and it is indeed quite easy to use. I recently discovered a great Youtube video on building a subwoofer box, that translates well to the HVAC box: http://www.youtube.com/watch?v=dyf7_SI0JUc The video references a box dimension calculator site that is quite handy: http://reaudio.com/box.php 

I plan on adding my baffle box model to the Sketchup library, as there were not any I could find therein.

I have already bought 3/4" MDF and 1" Certainteed rigid duct liner in preparation for building the boxes.