I needed to feed wiring cables through a 3mm aluminum plate and came up with this 40mm square grommet. There is an optional cover with a 14mm x 17mm cutout to feed cable through and dress up the appearance.
Printing on a HBP and raftless is a good combination for the base and the cover. The base is designed to friction fit into a 40mm square cutout, but you can add a drop of super glue if needed. The optional cover fits with a press tight fit onto the base.