Can an electrical subpanel be directly mounted to a hvac supply plenum?
Can an electrical sub panel be directly mounted to an HVAC supply plenum?


Yes.

Just don't be sloppy about it.  It needs to be mounted securely.  So consider using a plywood backing, or adequate fasteners such that it is not going to fall off if someone bumps into it.

Also, ensure that no air from the plenum is vented into the sub-panel to isolate the sub-panel from condensation; and that the sub-panel will not be exposed to temperatures outside its limits.