Turns out when you have a printer people find things for you to make, I was asked to make replacement spool caps for a sewing machine. I don't know the make of the machine just that the rod was 6mm so the opening is 6mm that tappers slightly to create a compression fit. Not all machines are the same so its worth checking the diameter on your machine. I suppose you can scale the model up or down to change the opening. The small is 25mm, medium is 35mm and large is 45mm if you scale the models that'll change of course. I'd suggest splitting the model and only printing the small if you're not sure if it'll fit your machine.