I see how you want to improve the airflow coming off the back of your trailer. But when you say you want the top to slope at 25°, it seems as though you have misinterpreted some of the information about basic aerodynamics here on ecomodder.
Here is my suggestion, do whatever you need to do in order to radius the shape. On top I would not try to go much past a 15° angle overall which is what my illustration shows here. Also radius the sides. This will give your modification the best chance to work well. If you just go straight to 25° (The red line on the pic here), I really don't think it will help much at all. Even if optimized however, I would not expect a 2 foot extension to return more than a 3-5% gain on mpg.
Hope this helps, I'd include more stuff but I gotta run now.
