Here's where I installed mine:
As with anything that's not spelled out in the plans, I spent a few hours figuring out exactly where to put them, but it probably doesn't matter much. I wanted them to be low enough to not be blowing directly on the back of the panel, close enough to reach without too much trouble (I probably have to loosen a shoulder strap to reach the far one), and far enough to give the air a chance to circulate somewhat.
With these doors, you can't really control where the air blows, but you probably don't have to worry about air blowing directly in your eyes :-)
I feel like people with the NACA vents and tubing and eyeball vents end up restricting the airflow to the point that they need to add scoops or windshield vents. I haven't flown in the mid-upper 90s yet, but one vent has always been enough to keep me comfortable.