Thanks, guys, for all the great input. One more question:

I've done a lot of online searching for good examples of a sheet metal equivalent to this ugly mess masquerading as a plenum (that WILL be completely scrapped and replaced, one way or the other). So far, I haven't been able to find anything even close. The closes I can find are bottom plenums designed for large rectangular ducts that come down the side of the furnace. Can anyone refer me to pictures, specifications, etc so I know what they SHOULD be specifying?

Are there any specific dimensions, proportions, minimum volume, etc that I need to look for?