Tested with Firefox/Greasemonkey 4 and Chrome/Tampermonkey.

I develop similar scripts for the Metacritic score, see cvzi/Metacritic-userscript
and for the Rotten Tomatoes score, see cvzi/Rottentomatoes-userscript

Supported sites and products:

• imdb.com
• metacritic.com
• letterboxd.com
• BoxOfficeMojo
• Amazon
• Google Play
• allmovie.com
• Wikipedia
• fandango.com
• thetvdb.com
• nme.com - New Musical Express
• itunes.apple.com
• save.tv
• wikiwand.com (only Movies and TV Shows)
• Radarr - you need to set your server address in the script header, default is @match http://localhost:7878/*3
• trakt.tv

Screenshot:

https://github.com/cvzi/Letterboxd-userscript/