My aerocap is covered on the outside with epoxy. Inside is bare Styrofoam. I figure that the fuel logs show about a 12% increase from no cap. It's slightly better than the performance of this earlier aerocap.
And this is why I changed shapes.
The air will conform as much as possible to the sides of a vehicle as the vehicle moves through the air. Air that wants to move sideways, will continue to move sideways until there is no more surface to guide it. Similarly, downward moving air will move downward as long as there is a surface for the air to cling to. When the air no longer has anything to cling to, the various air streamlines will interact with each other. Vortices will form as air moving inward to the vehicle's center meets with air moving downward.