I'm pretty sure that the aggressive wedge shape is essential, it literally splits the air. Shaping the "appendix" differently, could cause disturbances to otherwise completely clean undisturbed air. They have, in theory, the largest volume of clean air to direct elsewhere in all of the grid.. I wouldn't like to know the drag coefficients on that shape but F1 cars are closer to 1960s muscle cars when it comes to total drag.. But that certainly is one part of the car where drag is almost completely harmful, ie the device doesn't cause negative lift at the same time.. This shape just screams "drag" but the total effect just might be beneficial. Note, i'm being extremely nice towards Caterham, it is very refreshing to see something different on this part of the paddock, compared to Marussia which took ultra conservative approach. But there we have a good measuring device, Marussia conservative and safe, Caterham, innovative, but very risky.
The overall look is like they took the rulebook, draw straight lines to maximum/minimum dimensions and everything that could be taken off, is gone.. Don't know how heavy that thing is thou, it might be the actual reason why others have closed this part of the chassis. On first look my honest opinion was: "lol.. hmm... interesting.. that might actually work..."
Then again, how much are you gonna run in the clean air if you want to get close and catch the midpack... And that's my only defense on this design. Turbulent wake, things are very very different. This most likely works beautifully in the air tunnel and simulations.