How to save main project file in custom 'New project' package

[eny]

I have created a custom new project type as briefly explained in the wiki.
(Via an IDE package, not a project template.)

Everything  works fine, the project is created and the main unit is created and loaded in the editor. However all new files are kept in memory. What I would like to do is: force the IDE to save the main and project file in a target directory, that is created as part of my custom New Project dialog.
How can this be done?

The problem is not actually saving the files, there are many ways to do that obviously.
But letting the IDE know that my main form is (should be) stored in file x:\<my custom dir>\ufrmmain.pas and not in the default location for new projects.