I created a simple single ethernet wall grommet a few years back and it has been very popular. I needed to make an even bigger hole pretty that had quite a few cables running through it - enter the bigger wall grommet. To make printing it easy, I split it into two components; one that slips around the cables and the other just a simple box that slides into place and locks everything snugly together. The plasterboard I have is 9.5mm thick, so if yours is different, you just need to alter the grommet part to suit, if you need to fit more cables through the box, then you can make the box bigger easily too.