"square edges in a NA airbox to cancel standing waves"
AFAIK it's not the corners but the flat surfaces that reflect a very strong discrete wave, while a radius scatters its reflection - think stealth.
I don't understand why the shape affects mechanical strength, though. The classic "square windows doomed the de Havilland Comet" argument isn't relevant - that was stress raisers in thin sheet metal over hundreds of hours. The only clue I can think of is the surface to volume ratio, which places more stress per square inch on a square shape. Comparing a cube with a sphere of the same volume, the cube's surface area is only 80.6% (or (1÷(6/Pi)^.3333), or 124% of the load per square inch.