Use 4-40 or 2-56 hex screws with blind nuts. Just enlarge your existing screw holes and insert the blind nuts from inside the fuselage. Throw a little epoxy on each nut (being careful not to get any in the screw hole) to hold them in the fuselage. It is easier and faster to use a ballend hex driver than a screw driver. I use this method to mount the bellypan on my Vectorflight Edge 540. Very simple and highly effective.

Here a couple of pics on how I did it on my VF Edge 540. Hope this helps and good luck.