Hi phasmoides, one suggestion is to find an electrician and buy a short strip of the plastic hole guard ( 10cm would be heaps ), not sure what it’s called but the electrician will know what your after.

This is much thinner and should just about make the hole big enough for all your wires.

Then remove the rubber grommet and replace it with the plastic hole guard.

Once all the wiring is finished, seal the hole with mastic or selastic.

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