Paul,

Thanks for the compliments.  As for the plenum trick, the only advice I would say is support it from the bottom in a few places.  I used some foam blocks where needed till the part cures.  The tricky part is going to be connecting it to your current baffle.  If your plenum wraps over the baffle, then you have to run your mounting screws from the back/sides.  Sort of a pain, but you probably have more room than I do.  My first plenum was this way.  It worked, just annoying.  I would be tempted to attach some angles to your baffle where possible, then you are running the screws down from the top.  Much better.

 

Plenum pressure is tricky, and hard to explain.  This is my non-educated opinion, but it just makes sense to me.  First, picture an exponential line graph.  The far left 0 point is a plenum leaking all pressure out.  The far right end of the line is a water tight plenum with the highest possible pressure.  Lets say your starting point is in the middle.  As you reduce leaks you move up the pressure line, but you don't see the biggest gains in pressure until you approach the water tight plenum.

 

So in summary, yes build a plenum, it will help.  But, until you go full monty and seal your inlets to your plenum you will not realize the biggest gains of the plenum.

 

Tom McNerney