Aha, I think that’s the issue. I had the 10k resistors where the 470 ohm ones should have been and didn’t do the t/r thing to make it balance ambidextrous.

I’ll definitely have a go at making this cable, thanks for the link!

Or… does anyone know of a stockist for this sort of cable/box. I know it’s not a common cable to request but it seems like it’s not the first time anyone has wanted one.