@insl The ducts are too narrow to pass a fibre with its connector, so you need to splice it, but I couldn’t find a machine to borrow. In the end, I found a POF (plastic optical fibre) solution. It’s like TOSLINK, a plastic fibre 2mm in diameter, except it’s a pair, because you need both directions. There’s no connectors, you push it into a hole in the transmitter, so it’s easy to pull in the ducts. It works over at least 30 metres and it’s inexpensive.

@insl Much later I remembered that mechanical splices exist for traditional glass fibre, so I could have used this. But the glass fibres with jacket are still wider than the POF.