I am not sure the remain voting, people of Northern would agree with your definiton of a clean brexit.
After twenty years they will be back to more than 200 of these again.
county-fermanagh-northern-ireland-october-1980-cross-border-road-blocks-EECK5T.jpg
During the "troubles" about 18 of the 280ish road border crossings, were open. It would be impossible to police that as a Single Market border, so it would have to go back to most of them being permentaly closed.