Create a bare repo on the USB stick(/dropbox/Google Drive/random folder) and just push to the USB stick.
That said, I haven't tried this lately, maybe it's gotten more robust over time. But historically, even a bare repo on something like Dropbox has issues.
I fully agree that this is a niche use-case.
This wasn't what I was talking about, I meant that you should create a bare repo and push to it, not that you work directly in a directory in Dropbox.