As Daniel Miessler pointed out it's possible to obtain an RSS feed for a youtube channel. However, you need to replace channel-external-id with channelId in his blog post.
Instructions
Visit a channel page
https://www.youtube.com/user/PewDiePieView the source and then filter for
channelIdAdd the
channelIdat the end of the string below
https://www.youtube.com/feeds/videos.xml?channel_id=
Feed Generator
Enter a channelId:
https://www.youtube.com/feeds/videos.xml?channel_id=UC-lHJZR3Gqxm24_Vd_AJ5Yw