I was also using Stuffit (5.5) originally, but I just gave it a try with good ol' standalone MacBinaryⅢ and that worked too. It should go like this: https://imgur.com/a/YKd4Zvs (first three screenshots)

Once it says Done you can trash the installer. There will be a new folder in the same location as the installer, and within that folder you can run `MacRelix`.

The last two screenshots are me using the `sbin/upgrade` script from within the first Relix flavor to fetch a second Relix flavor.