I’ve updated the script and posted it below using youtube-dl instead of Downie.
I have not tried this myself, so let me know if it doesn’t work and I’ll take a closer look.
(Note that there are still some references to Downie in various filenames, but that should not matter.)
I’ve also put it in a proper gist, so it should be easier to download, and easier for others to fork if they wish.
Note:
Despite the name of the gist, you should still save this to /usr/local/bin/downie-get.sh or else you’ll have to change these two lines in the plist:
<key>Program</key>
<string>/usr/local/bin/downie-get.sh</string>