This picture had what may turn out to be a life-altering effect on me. I'd stopped thinking about fastening composite materials, but it reminded me. Basically you bore through all three layers and counterbore through one skin and the substrate. A stepped washer shape grips one skin on both sides and the full material on both sides.
I went to a local fastener supplier to see what they had. It looks like I will have to take a high-grade washer and machine the step myself.
In case you missed —my 'World's fastest motorhome' project has morphed* into a Lakester to run in the 36HP Challenge.
*competion is too stiff--> 138mph.