It is a pain - I've found 3 formats for users, channels and playlists:
https://www.youtube.com/feeds/videos.xml?user=<user name from url>
https://www.youtube.com/feeds/videos.xml?playlist_id=<id from url>
https://www.youtube.com/feeds/videos.xml?channel_id=<id from url>
Sorry for the repeated post, but if you have an account, you can subscribe to the channel and then get an OPML file with all the feeds from the Subscriptions Manager page.