The flat surface of the usb-pass
through is around 7.5 to 8 mm. There isn't much choice in the ring-magnets, but I think the 5 mm diameter with 2.5 mm hole would do fine (so that's 44600-1 N42 ring magnet, D5xd2.5x1mm ). Perhaps safest to put a sliver of isolation-tape around the outside of the circle, just in case it gets against the side? (Not sure if that would be easy doable - but would probably be the safest option I think - or maybe something that can be painted on, plastic paint for instance?).
The link you found is a nice one... but I doubt it will be usable. First off it's a US-site, and just too many US-shops tend to not want to ship internationaly (so that would be bad luck for me but not for you of course
) but more importantly: look into their order info link: "Mininum for purchase order is $100 not including freight & taxes." ... I doubt any of us want a load ofthose ring-magnets worth $101...
But anyhow, it does give an idea what to look for elsewhere . At least we know an answer is out there