Woohoo! Boat tails are so awesome. What an excellent idea to involve the class.
First: The top looks great.
On the bottom, I wouldn't go so "fast". The reason: you're not dealing with clean flow on the bottom of the car as you are on the top, so the angle can't be as extreme. Bottom flow is already turbulent, so more prone to complete separation if you arc away from horizontal too aggressively.
I've tuft tested the bottom surfaces of both my Insight & Metro/Firefly tails, and observed attached flow on both by not exceeding ~13 degrees on the Honda and respecting a much more conservative 6.5 degrees on the Metro/Firefly. (Arguably the Honda has a much cleaner underbody design, which is why I went steeper with it.)
Another critical consideration to keep in mind: the "transitions" from the side to top/bottom surfaces. Use generous radii there, not hard corners, to minimize the chance of "tripping" the flow into vortices.