Paul - in your mock-up for the -7A you show the intake runners protruding
halfway into the box-shaped plenum.  is there a reason for that vs. having
the bell mouths pretty much flush with the wall?  it seems to reduce the
volume of the plenum and is not intuitive that air swirling about the runner
bases inside the plenum accomplishes anything.    kevin

It is an attempt to get as much volume in there as possible given the
height restraints. Nothing significant about it other than that.

The plenum is supposed to attenuate the pulsing. The larger the better.