Originally Posted by
FlyFast
You said that this is not a structural member, I hope you've checked with Zenith on that. It could be that the riveted structure serves as a stiffener. On the other hand, if it really is just a cover, non-structural fasteners would be okay. It is always best to check with the designer on these things.
Where will the back of the fasteners be after completion? If they will stick out the bottom of the airplane, you'd probably want rivets.
I agree with Aaron, that putting some sort of nut on a screw will greatly reduce the wear on the sheet metal.