I certainly agree with Neal regarding using Avex rivets with a properly sized washer for fiberglass. Makes a lot neater job and the Avex rivet will pull things together very well. I am not sure that using counter sunk screws is a good way to secure panels that are installed and removed alot. I still prefer Dzus fastners, if you use the flat head style with the long under cut you can install the fastener through the panel and then slip a small "O" ring, I like to use viton material, over the fastener and it will retain the fastener to the panel. I worked on several GTP cars, Lola T600 and Indy cars, March 83G, that had a lot of panels retained with flat head screws and you soon found that you needed to have your 10-28 or 10-32 tap in your pocket to fix the threads that you screw up trying to get the fastener in, and you also spend a lot of time putting a taper on the screws so they can find the thread.
Rex