So not shown is the method of attaching the flashing permanently. It is too cold here to apply glues and paint so this step will wait till spring. But what is planned is that I will use gripper on the foam surface and flashing, where it comes in contact with the foam, just before I clamp the pieces and pop rivet them together. This will help with better grip and weather tightness as well. If I was to have too large of clearance behind the flashing in the cut out I might also want to apply some GS. I would apply the GS after pop riveting, but not before removing the clamps. I would do this by drilling a hole in the foam large enough to stick the GS nozzle back behind the flashing in the void area. Now you can give it a shot of foam and the clamps will keep it from lifting the flashing up off the finished surface.
Shots of how the flashing allows for hard points for the installation of the vent. Also, by pop riveting the two angles together you make a channel which makes for a strong support of the foam in this area.