After re-watching the part of Scott McDaniel's video on fuel tank, I started by cleaning both of the cap flanges and the skins with acetone, applying some masking tape to protect the inner taper, and then mixing up a batch of sealant. I then spread the sealant along the mating surface of the flange, leaving a ridge of more sealant along the center of the contact area and thinning it towards both the inner and outer diameter edges. A couple of clecos at the forward and aft rivet hole kept the filler cap in place during the actual riveting.
Since I didn't have a short enough yoke for my hand squeezer, I had to use the pneumatic rivet gun. Fortunately, the riveting went quite smoothly, and I got all of them set perfectly on the first try. After some cleanup of the excess sealant, I applied a little more over the shop head of each rivet using my drinking straw trick, and then fileted the sealant seams.
What's next? Time to set the skins back in the saddle fixtures and do a final test-fit of the ribs before riveting them in too.