a had a corsa once but it met with a wall, still it intrests me how aerodynamic i might have made it if i'd still had it
since the prius is a bit of an icon when it comes to good earo i made a quick comparison between the two... the sketches are not to scale but it's interesting to see that the overall proportions of the two cars match quite well, only as as apparent in the first comparison to be as aero as the prius the corsas roof would have to drop considerably wich could make it look more like a CRX
when looking at the roofline curve and continueing prius style where the opel roof stops you can see it shoudln't be to hard to make a kammback that continues that line.
it would seem that your kamm angles down a bit to steep. you might get better results with changeing the angle wich couldn't be that hard to do i imangine.
also your car has these gentle curves at the back, generally a clear cut edge will promote cleaner saparation. perhaps small spoiler strips to the sides could cure this...
could make a difference in aero too