One thing that I did that made sanding a little bit easier was to make a jig to hold the plenum in place which can be clamped to a bench.
Note the additional pair of holes above the bolts on the right side. These allow the plenum to be rotated 180 degress. It also can be flipped over and rotated 180. The additional wood piece down the center was added for increased rigidity and for extra thickness so that the bolt heads could be recessed. Recessing keeps them out of the way when the jig is in the horizontal position.
One of the 4 horizontal positions:
The side allows the plenum to stand up vertically:
Elevated (to reduce bending over):
Counting the elevated ones, there are 12 different positions; useful when delving into those crannies...