I bought a Lixhult metal cabinet from Ikea and wanted to place backup hard drives inside the cabinet. In order to ensure proper cooling of the drives, I needed to make some air holes in the front of the cabinet. These air hole grommets fit over the holes I made in the cabinet sheet metal, allow airflow, but hide the sharp edges around the holes. There are two air grommet designs: small for the front (12mm) and large for the back (31.75mm).
The holes on the front of the cabinet were made by first drilling small pilot holes and then using a stepper drill bit to create 12mm diameter air holes. You'll notice there's a vertical area where I did not place grommets. That's because on the inside of the cabinet lid there's a vertical bar which provides rigidity to the cabinet door and I wanted to avoid getting too close to it.
The grommets are then pushed into the holes.
Both grommets were designed in Fusion 360.