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Make sure its roughly the right size, as you'll be at the upper end of what's available on Amazon. If you measure the height and thickness of the mount you should be able to calculate it. I think the diameter should be bigger than (2*height+2*thickness) / 3.14. My guess is it's about 3cm high and half a cm thick, in that case you'd need a diameter of at least 22mm.
Tijmen-
Thanks, I've been looking on Ebay as well, luckily there seems to be enough variety in diameter :)
I'm also thinking of using a bit of inner tube to put some extra protection on the corners underneath before finishing it off with the heat shrink tube, which would increase the diameter some more - I'm sure I'll find something that suits. Thanks again for the suggestion, it hadn't occurred to me at all.
SwissChap
Ooh that's a good idea! I've never used it before, but that should work quite well actually. Thanks! Any specific recommendations or should I just buy anything that says 'heat shrink tube' on Amazon?
@Sumo
Well ideally I'd like to avoid doing anything that can't be reversed, or that will impact the finish. If I knew I wanted to go fixed forever on that frame I'd just remove the mount entirely, but I'm pretty sure I'll be putting some gears on it in the future, so I'd like to keep the mount itself as it is.