I needed a simple through-hole grommet for cables for my makerspace surfaces, which I made from 1 3/8" interior hollow core doors.  Because the doors are hollow, I wanted the grommet to be secured from both sides of the surface. This snaps together with fingers that can subly flex on one side and a snap-in ring on the other. Nothing fancy.

Printed in black PLA, but there is nothing special about the material that is needed. I did use 6 shells to try to give the flexible snap-in fingers more strength, but it might not be necessary.  I went with overkill, not optimization.

To assemble it, I put the top piece through the hole and dropped a pair of scissors through the center.  Laying under the desk, I aligned the other side and held the scissors through the center, pulling the top against the door while pushing up on the bottom piece.  Once held together, I was able to reach through the grommet to pull it flush with both sides until it snapped in place. Took 30 seconds.