This is the form blocks holding the skin in tight. One of the problems that I ran into while making the skin fit was how to get it even on the tail to the fuselage. It finally dawned on me that the thing to do was to fold it over like it was on the tail, and put the bottom of it on a belt sander. This gave me a nice straight bottom on the tail skin with a nice small gap between the skin and the fuselage.