Here is what you want to try and do. You'll want to radius the corners of the tail extension as much as possible to help you in a cross wind. My angle may be a bit too steep, you certainly would not want to go steeper, going at less of an angle would probably get nearly the same result.
Notice how the shape starts out "Blended" into your trailer? Then it slowly curves to an angle, this is what Aerohead meant, I believe, when he said you don't want flat sides creating an abrupt transition.
I have about a 13° overall change and the back of the shape is at about 21° before it cuts off there. FWIW
Here is a link to a bigger slightly more detailed version of the same drawing as below.
Bigger Pic Of Trailer Kamm Dwg
