I wonder if the most obvious, and least unpopular, tax to raise the Single Payer money would be a payroll tax — of similar magnitude to what employers now pay for their workers’ healthcare. No net cost there; employers are unlikely to have a serious objection to paying the same in tax that they now pay in insurance premiums.
That will only go far enough to cover those already covered, of course, so there would need to be a topup. That’s politically harder. But say you charge those employers 15% more than they’re paying now; another payroll tax for the smaller employers who DON’T currently offer insurance to their staff; a small increase in social security contributions on employees — maybe it’s doable.