The tests can be free. They can remain available to anyone wanting to retake the exam. They can even be delivered verbally to avoid the 'literacy test' arguments that leftists will almost certainly fall back on.
But there should be a small hurdle to voting. A 100 question civics test where you have to be able to get at least a 75% would weed out millions of leftists. Hell it would literally save states like California
Are the Federalist papers in force? If it's just history value, add the anti-Federalist papers too.
Also require voters to be net taxpayers. And to demonstrate at least basic policy positions of candidates they're voting for or against.