"Bonus points for wanting transparency and a system that's open source."

Transparency is not a bonus, publicly verifiable elections are a requirement for a healthy democracy to ensure power is legit. Open source does not really matter. Even if it's open source, it's hard to assure the public that "that open source software you downloaded from github" was actually running in a counting machine and nothing else (i.e. a custom made virus) at the moment of counting.