GTK main window calling a main window

[DennisT]

I've been googling all day and can't find how to do this. 

I have my main Python 3 Gtk+3 window.  I want to create a overlay window (smaller than the main) that is centered on the main window.  The overlay window will have focus - users cannot select the main window until the overlay is closed.  The overlay window will consist of a grid container(uses entire overlay window).  I will open and close the overlay as needed.

Can anyone point me in the right direction to do this?  I tried a dialog but couldn't get it to accept a grid (box only?).