Carolie flew home this morning to visit her mother and brother, so I moved the parts home so I could work in the warmth of the house. I mixed up a small amount of ProSeal and made the fillet between the "ribs" (I don't understand why Van's doesn't say to put it along the seam to the left and right of the ribs, too). Then I put some on the flange of the firewall and riveted the "shelf" in place.
No problems. No mess. It's like I've done this before.