Then why not have your MC use a local Library's computer. I believe most of them have computers that are available to the public for their use. I don't know how many would have internet access but some do. Save anything needed for later activity on usb devices or some other media. As long as the MC doesn't use the same library regularly and doesn't set a traceable pattern of library usage it might be hard to narrow it down to a particular person.
Being on a USB Drive doesn't fully cover you, there are ISP logs that could potentially "get you" via TCP/IP protocol stuff relating to NAT specifically and MAC Addressing. Likewise, depending on the public Library, thanks to MAC address stuff depending on what level of detail is getting logged, they could narrow it down to specific computers at specific times. And if the library keeps logs, or has security camera footage....
Anymore, the "better" option would probably be to avail yourself of free public wifi, but when you use it, make sure you're not using the "stock" wifi that comes with your computer, and make sure that activity does only go to the USB drive.
Or for a slightly easier option, find an old wifi device that can't be tied back to you(don't keep it, don't advertise you had it), record what its MAC address is/was, and before you connect to "public wifi" make sure you switch your wifi device's MAC address (via software) to that specific address, and remember to change it back once done(This is actually something that can be done through the stock Windows interface for that matter). So you're not running around with two physical wifi interfaces for a lone device. :)
There are programs "out there" that can generate valid MAC addresses at will for you as well. So in that respect, you could have a character go that route, so their illicit acts never happen from the same MAC(but at the very small risk you might eventually come up with a MAC address being used by a device on your specific sub-net, as unlikely as that may be). But that is really wandering off into the weeds on tech details. :)