You can follow the instructions shared by Apple in the support document, [MacBook Pro: Bottom case foot replacement DIY instructions](https://support.apple.com/kb/HT202450).

There is absolutely no need to shell out money to replace the entire bottom panel. However, the article explicitly mentions that the DIY replacement kit is not available for MacBook Pro with Retina display computers. But the procedure shared appears quite straightforward. 

As apparent from the article, glue can be used to stick the feet to the bottom panel. On the other hand, [this](https://www.youtube.com/watch?v=8VIHAae8N2Y) Youtube video gives a good demonstration of how to remove the bottom panel and place the bottom feet the recommended way.

**Note**: I have had a similar situation with my Late 2013 15" retina MacBook Pro. I have used glue to paste the falling off feet to the back panel.