Too bad about that square topper, but I hear you: you're pressed for time and don't want to mess around with fabrication.
Unfortunately, there's no commercial aero topper available. Not yet, anyway. It'll come some day.
You can improve the rear of the bed/square topper combination mainly by extending the overall vehicle length to taper the sides and the top. Think:
from:
http://ecomodder.com/forum/showthrea...utt-21071.html
But again, it would require some fabrication skills.