Install older OSX on separate volume.

Dear all

I want install an older OSX version on my iMac13,2, preferably Catalina. I set up a separate disk volume. The problem is to find an installer that will work and is not blocked by Apple. Anyone?

Alternatively, I could install and older unsupported OSX. HoweverI could not get this to work with OCLP. I believe only one unsupported OSX can be installed at a time. I could be wrong and am open to ideas.

Thanks

Anton