I want to make a hole in my top tube bag to run a cable through for cables and charging duties, I am not too worried about it being waterproof as it will be tucked away somewhere underneath.
Does anyone have any suggestions where to find the type that certain jackets have, like a cross cut in a rubber disc?
I did consider using a simple 20mm electrical cable grommet but I don't think the sides are rigid enough to keep it in place.
I could reinforce it with an eyelet and make it fit?

cut up inner tube ?
possibly double layer for more rigidity
lsyer of duct tape, etc and make your own cuts
Glue some patches on either side they are thicker ?
If you wanted it there permanently I would just add a blob of sealant.