Bill,

I need to see a close up and a further out picture for orientation.  I also have Standard NACA ducts, although I slightly altered the outline to meet specs and squared up the ramp to sidewall and sidewall to fuselage joints.  I have 1/8 inch mesh hardware cloth at the plenum entry to keep out wasps, eyeballs on the plenum itself to cool my feet, eyeballs on the panel for face air (fed by 1 in tube), and the passenger plenum also has a 1/2 inch tube tapped in for radio stack cooling air.  I get air coming in weakly with just the prop running and plenty during low speed flight right up to the AOA warning - Angle, angle push!

I can't imagine one could get air sucked out unless the cockpit is pressurized or there is no way for incoming air ro exit (my friend's E-Racer pusher had this problem).  Generally, air should be able to get behind the panel, down the elevator tunnel and out the elevator opening in the vertical stabilizer

Good Luck!

Scott Krueger