Seems like that would just be a matter of non networked machines under physical and digital security. Pretty much the same way, you'd ideally handle paper ballots. There's no way to make a "hack proof" voting machine. Microsoft has proprietary software on a good number of them actively lobbies against open source. (no real shocker there) The problem is we often rely on the company that manufactured the machines to audit them. It centralizes way to much power into the hands of corporations and doesn't allow for 3rd party review after an election.
i view corporations as far more trustworthy than foreign nations. there needs to be better review policies and what-not, but open source? too far, too decentralized for something so important. it'll be a ticking timebomb until someone stuxnets an election.
That's what foreign nation states do. Every cycle.
Exclusive: Russian Hackers Attacked the 2008 Obama Campaign
On the other end of the spectrum it would be extremely unlikely a small closed source dev team would ever discover they had been successfully exploited. We can't even rely on FBI agents to behave professionally and show no bias on the cases they're working. Why would a traditionally liberal dev team working out of Redmond or Silicon Valley be any different?
personally, i strongly disagree. i hate to poopoo on the open source parade but while it allows peer review, it also gives direct access to everybody to hunt down where the weak points are. there is not and never will be a perfectly secure system, there are only obfuscation techniques to make it more difficult and they start with not handing foreign nations the source code of our democracy.