Um....maybe just get a 1/2" square tube and fit it between the drivers side-passengers side roof rails and....with a rearward angle, weld a short round metal tube onto the center of the square tube. Drill and tap a hole in the round tube for a set-screw (to secure the flag post.
Figure our a way to clamp the square tube to the roof rails for a secure fit of the whole unit.
Or maybe fabricate some kind of "L" brackets that can be bolted underneath the rear roof rail mounting pads, then weld the square tube (as mentioned above)....weld it in between the L brackets.