I used a twinwall poly panel for the roof with an internal milky twinwall that is a bit more flexible for insulation and light dispersion.
I'm in a cold climate so the second layer probably makes more sense for me.
Definitely install an exhaust fan to remove heat in your hot climate.
I do have glass walls with a clear plexiglass interior layer (saving heat again) which I love, but don't see any use for a glass roof, the poly panels disperse light nicely and I don't have to worry about glass falling on anyone if a storm blows something at my greenhouse.
Again, the more vents you can open in the summer the cooler you can keep the greenhouse.
