Why do you need a duct? Most bathroom vanities, for example, are wide open from a slot in the floor to the louver on the kick panel. Just close off the compartment by any convenient means and call it a day. Staple some cardboard in if that works.
And parts from a big box store should work just fine. A common 2x10 or whatever size duct could be laid right in ...
This may or may not apply to you, but you have a newer system so there's a better chance of this being the case.
Some air handlers have blowers that can operate at more that one speed, and the zone controller that you have can control these speeds (section in red under #6):
Zone Controller Manual
When multiple zones call for heat or cooling, the fan ...
Could it also be that the walls were patched prior to selling, and the cracks, which were already there before, resurfaced? Just as @DanielGriscom suggested, a hairline crack on an already patchy, uninsulated wall, in front of the heating pipes, points at "it's nothing".