How to declare global variables in SSIS 2017 that are visible to all packages in the project

I created a project in Microsoft Visual Studio - SSIS 2017 that contains 32 packages. 

I have 4 variables that I would like to declare that needs to be global to all packages.   I can create variables that are visible to one package. 

How can I make these variables global to all packages?   Is there a simple way to do so?

Thank you