This bolts on top of the lower runner assembly, right?
Depending on how nerdy you are, get yourself the trial of SolidWorks, and 3d print that s.o.b. out. Heat shouldn't be a problem there.
I would estimate your material cost in aluminum to be almost $175, and then the machining and welding your design requires... So 3d might be a viable alternative.
You can use Rivnuts or Heli-coils for all your fastener points. As a bonus you can do the right thing and print out trumpets that extend off the rounded plenum walls- no corners.
Btw, I had thoughts of doing this before the itb project...
Matt