This seems to be the most functional YouTube Downloader extension available right now. The only downside is that instead of starting the download within the page, like you'd expect, it instead opens a new tab for youtube4kdownloader.com to download it from there. However, the download does then start automatically without you needing to do anything, so all you have to do is close the tab after it starts.
I'm no programmer, but it seems like it shouldn't be too difficult to make this download process all happen within the same page/tab. Otherwise, this is great.