You can download 'early release' plugins here, before they are included in the official Redshift 'production builds':
https://redshift.maxon.net/topic/31230/houdini-solaris-plugins-custom-builds [
redshift.maxon.net]
In the download there are plugins for both the 'Houdini' and 'Solaris' folders. On windows add these to:
C/ProgramData/Redshift/Plugins/Houdini/ and .../Solaris.
If you are using a more recent Houdini build than the plugin is designed for, it's likely it will still work.
At the time of this post the latest Redshift early build plugin is for Houdini 20.5.278, but the latest Houdini build is 20.5.307.
Having copied them, simply rename the 'Houdini' and 'Solaris' plugin folders to the build number you are using - eg rename "20.5.278" to "20.5.307".
Houdini will throw an error on launch warning you of the plugin mis-match, but for the most part you can ignore that and things will work as expected.
The Redshift 'production build' plugins get updated as new builds of Redshift are released.