And this lead to essentially re-inventing the muffler.
I slid the 125 mm ID SS pipe over the 65 mm ID outlet-with-a-12 cm-gap contraption, insulated (spaced, centering) the ends of the larger tube (maybe 5 cm wide) around the 65 mm tube, sealed the 70 to 125 mm radius openings with tape, and perfect smooth air now comes out the 65 mm outlet.
In other words, I think that a cheap 2 1/2 inch performance muffler can be stuck onto the blower fan and it will give easy-to-measure smooth air.