Summary

Grommets are used to protect cables/wires passing through panels.
Stepped grommets allow you to suit a range of cable diameters using a single part because you can cut the diameter of this grommet to size during installation.

Only one company sells them and they price gouge pretty bad if you buy in small quantities. So here is a new option!

Printed using eSun brand 'eLastic' filament: https://www.amazon.com/eSUN-eLastic-Flexible-Printer-Filament/dp/B01A4WP4AY/ref=as_li_ss_tl?ie=UTF8&qid=1496509079&sr=8-1&keywords=esun++filament+elastic+1.75&linkCode=sl1&tag=engin09-20&linkId=64774c7b4270b6785df41679fcd6deb0
(I also tried using a semi-flex material but that came out too stiff. High flexibility is needed.)

Pictured- two off the shelf grommets at left and an example of all 3 of these printed models. Notes:
In the name of each stl file shows the part OD which is the panel hole drill size. The ID is the maximum cable diameter it can support.
If you need to scale the model pick the closest one in size from these three, because they were all altered to have 1.5mm thick wall.