I did finish my plumbing and fired up the black box last evening. I placed a a 3 foot square sheet of aluminum directly on the plenum (no spoil board). Using only one motor in the BB, the sheet was not securely held in place. With both vac motors running, the hold down was solid. Not sure what the result will be when I install the spoil board, but I'm pretty sure that running only one motor in the BB will not be sufficient for most applications.
Your installation looks great.
