I will, gluing the outside flange to the stucco to make it water tight (will go back tomorrow and run a bead around the edge to really seal it up).
I built a frame on the inside and once the glue dries, I'll go in from the inside and screw them into the wood framing to secure it. I have some bungie cords pulling the gable vent inwards from the attic, but the ladder is just a safety precaution to apply a little extra pressure inwards.

