The answer can be found in
http://www.usfirst.org/robotics/2006...s.htm#Section3 where the blue prints of the center goal are.
In drawing 2 of 3, it show the side panel and the holes where the front panel will be fastened to. It is not noticable from the Arena pictures, but it is there all right. Some quick calculations indicate the angle is about 11 degrees from the vertical plane. If you want to find the exact angle, look into the blue print file.