Since the objectionable symptom is the hood skin vibrating vertically at certain conditions of engine idle, before pondering a permanent fix why not try a temporary one to discover what gets rid of the vibration?

Improvise some shims of multiple card-stock paper or non-corrugated cardboard that fit tightly enough to not fall out in temporary use. Then drive around to test whether the hood vibration is gone.

Tech explanation: It's likely the vertical engine vibration at idle and the natural vertical vibration of the unsupported hood are in resonance. Adding firm contact of the stiffening rib below will raise the hood's vibration frequency higher than before and avoid resonance with the engine motion at idle.