The issue with the rivnut is the flange - the cover plate you are attaching is going to sit up on the flanges of the rivnuts and not be truly flush to the structure below. Though one of the members of my local chapter demonstrated a dimple die he made just for this purpose - rather than the angled dimple we are familiar with for countersunk rivet heads, this countersinks a washer-shaped circle of constant depth into the surrounding material. Install the rivnut into the countersink and the panel being attached gets held down flush to the surrounding metal. I'm not sure if a dimple die like this is available commercially.