This userscript will show a film's Letterboxd rating on its IMDb page alongside the IMDb rating. Clicking the rating will take you to the film's Letterboxd page.
For upcoming releases with no ratings yet, an attempt will be made to link to the Letterboxd page if a) the film exists and Letterboxd; and b) the film's rating bar is shown - this usually means it needs to be getting enough attention to trigger IMDb's Popularity Meter to show.
You'll need a userscript manager installed in your browser. I personally use Tampermonkey, but others like Greasemonkey and Violentmonkey are also available.
Option 1: Install via GreasyFork.
Option 2: If you don't wish to use GreasyFork, simply view the .user.js file in your browser. Your userscript manager should prompt you to install it, if not you can copy and paste the contents of the file into a new script and save it. Do note that repository URLs and file/directory names may change in the future, so installing from GreasyFork should be the preferred method.
IMDb film page with Letterboxd rating added:
Upcoming release with a Letterboxd link added (note that the Popularity Meter causes the ratings bar to be shown despite not being available to rate - this is necessary for the script to insert the correctly styled link):
See CHANGELOG.md