Thanks for sharing. So the single speed chain you used is 1/8" wide, then? Single speed chains also exist in 3/32" width, which is about the same width as an 8/9-speed chain, so it should not rub anything. But I guess the difference between that and an actual 9-speed is small, and they will work equally well in practice. Perhaps the single speed 3/32" is a little stiffer side-to-side and less likely to derail.