For a large irregular hole, first cut a piece of neoprene slightly larger than the hole to provide a flange. Cut a suitable hole in the middle. Cut a slot if it needs to be slotted over an existing cable. Place it under the hole. Retain temporarily with masking tape or similar.
Then do the same again for the outside. Line up the slots. Run a thin bead of superglue on the neoprene just inside the hole in the panel. Place the two together and hold until the superglue sets.
Remove the temporary masking tape and you have a grommet.
