I prepped the aft baggage floor, the doubler plate, and the reservoir attach-angles for assembly, and prepped and primed the attach angles. Because the reservoir will sit flush on the floor, the blind rivets that attach the floor to the airplane might rub on the reservoir. I decided to try putting some washers under the angle pieces to lift them up far enough to clear the rivet-heads. (I ground them down enough to clear the nutplate holes.) In theory, this would work, but after thinking about it for a while, I realized this might put undue stress on the four small contact patches under the washers. Therefore, I'll simply countersink the floorboard and install flush blind rivets to attach it to the airplane. Problem solved.