A while back, this article was written here on the Discourse forum: Sharing custom filters using XML
Was hoping for some feedback so it could be converted to a Wiki guide. But it sort of faded away without much response.
Even if you do not choose to experiment with XML in the Custom Filters (where error are pretty innocuous), you should probably learn more about the Gramps User Directory. (There’s another article where we share info about where to find various support files in the different OS families the run Gramps.)