On all of my belly fasteners that went through the Fiberfrax, I made small .063 aluminum spacers. That way I had a solid Ti-al-al connection all the way through. With the Fiberfrax laid on top, it was easy to locate these spacers, and using an Xacto knife, cut out the Fiberfrax to clear them.
This is on my RV-3B, using 1/16"Fiberfrax and .020 Titanium for the belly overlay. The airframe itself has the original belly skin there per the plans.
On the Ti edges, I stopped the Fiberfrax maybe 1/2" short of the rivet line and riveted the Ti directly to the aluminum belly. Worked out very nicely.