Earlier quoted context omitted.
"Please timebox to an hour" is a bit dodgy though. If one candidate really does that, but the rest give it an extra 2 or 3 hours, that 1 hour candidate will probably be at a disadvantage.
I often ask for them to version control and send me a zip of the git repo, so I can look at commit histories. Yes, they can fake that, sure, but I can normally tell a lot about a dev from that, and actually if I see 4-5 commits over an hour perhaps with some wrong turns, that pushes the candidate up in my estimation than somebody with 1-2 commits that don't tell a story, but does show a complete solution - which is m…
What you're really asking for is a candidate to spend extra time inventing a convincing git history.
I really think the only way this actually works is if the problem is on a page which you can only view after you start a timer or something.
Or do it on site and watch them.
This is all high pressure but if you really want to time box, there can't be a way around it.