I may be a lowly law student, but I'm pretty sure that David Rivkin and Lee Casey are flat-out wrong when they say that a federal mandate for health insurance would be unconstitutional.
I think that it's terrible policy, personally, and that a Medicare-for-all single payer system would be a drastically better system, but their reading of the Commerce Clause in light of our currently existing system of mandatory old-age health insurance, coupled with the existence of the Department of Health & Human Services and the CDC, and the fact that the federal government has already done years of research into the impact of the uninsured on interstate commerce seems to be placing a healthcare mandate in some sort of parallel universe where the government isn't already accomplishing many of the same goals in different ways - and doing so in ways that haven't been declared unconstitutional.
Also, if you accept that a direct insurance mandate is unconstitutional, there's no reason that the federal government couldn't pull a South Dakota v. Dole and simply use federal Medicare/Medicaid purse strings to force each state to institute a health insurance mandate. I'd be very interested to see, though, if Rivkin and Casey would argue that Medicare is unconstitutional, which would cause aged, infirm revolution in the streets. It's been a frequent contention of Medicare opponents since the inception of the program that there's no constitutional basis for it, but it does create an interesting question: if a mandate passed, and the Supreme Court found that health insurance mandates were unconstitutional, would the abolition of Medicare lead to a constitutional amendment guaranteeing a right to health care? In the era of baby boomer retirement, I think such an amendment would almost assuredly succeed, which would pretty much decimate any opposition to universal health care in America.