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This works - and as a bonus the slots are kinda rounded at the bottom:
Get a feeler gauge set (cheap, from auto parts shop) and some 400 grit wet or dry sandpaper.
Find the gauge slightly smaller than the string diameter for that slot - the extra thickness of the sandpaper will give you an appropriate slot width (slightly more than the string diameter).
Wrap the sandpaper around the feeler gauge and cut, frequently changing the position of the paper on the feeler gauge's edge.
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