I've put together 10mm plastic spacers which do the job but I'd like something a bit more solid until it goes into a new chassis sometime later this year/early next year.
Ideally I'm after something in black nylon/delrin etc, but could always paint them, 20mm diameter and 30mm long with a 5mm hole through it, something like this but with a smaller hole,

I'd need two of them, if anyone knows where I can find something similar that would be great :)