http://dosdude1.com/catalina/

macOS Catalina Patcher

The pre-release of Catalina Patcher is ready for use! It can be downloaded here. (Current Version: 1.0b15)

Important Notes
This program should be used for testing purposes only.
There is no APFS patch included with this tool at this time. This means that it can only be used to install Catalina on machines that support APFS booting natively.
Be aware that this is an initial patcher release, and will probably have some bugs.
You MUST run the following in Terminal on the machine you want to boot this installer drive on, before booting off the installer volume, otherwise it may not install successfully:
sudo nvram boot-args="-no_compat_check"