Welcome to ecomodder cons and best of luck with your project. Several people on this site have constructed some really nice aerocaps using a variety of materials.
MetroMpg is right on about the key being a smooth transition off of the roof of the cab into the ideal angle of the roof of the cap. Since to sides of an aerocap become smaller as it moves rearward, I don't know if angleing the sides will have much benefit.
Here is some video of the Aerolid showing the flow of attached air along it's surfaces. This is just one example to follow in determining the shape of your aerocap. Even though the sides are not angled in, the air remains attached, along the sides, almost to the rear corners.
Bondo