Hi Werner,
I looked though some original Ford factory photos showing 1928-1929 Roadsters with top boots and found the following photos that might be of help. I have cut out and enlarged the top boots so that you can best see how they look and fit and also the position of the fasteners, etc:
Note not only the position of the various snaps, but also the two slots where the roof tie-downs protrude through the top boot at the two top rear edges/corners of the boot.
Hope that helps.
mit Ford Grüße aus den USA,
Brad
PS: Are you touring in the Harz with the Model A group this week?