The method 2 of your referenced answer should still apply.
The Crux library is single module library.
You can try and create a new folder "Crux" in your project's main directory, adding the content of the library there.
I would recommend to clone a fork of the Crux repository instead, through a git submodule add if you want to make changes in it.
git submodule add -- https://github.com/me/crux Crux: that will create the Crux folder.
(replace "me" with your GitHub account name, after forking the original chimbori/crux repository)
Add a 3.5.0 branch to the v3.5.0 tagged commit, in order to do a git submodule set-branch --branch 3.5.0 -- Crux.
cd Crux
git switch -c 3.5.0 v3.5.0
git submodule set-branch --branch 3.5.0 -- Crux
Your settings.gradle file would then include:
include ':app', ':Crux'
Do a gradle sync and ensure that the build is successful.
Edit your project's build.gradle to add this in the "dependencies" section:
dependencies {
    //...
    implementation project(':Crux')
}
That should allow you to edit the Crux library code as you want, directly within your project.