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Amazon Basics TOSLINK cable. Normally £14.89 for a 35ft cable, but it is currently reduced to £6. Some 3 foot cables are more expensive. Not sure how you would judge the quality.
Rated for being buried in a wall.
What can I say? Saw it, ordered it, got it, and plugged it in. It works. Even with 30 foot of it still wound up behind the tv.