[Sven7]

Front end looks good- just make sure those corners on either side are rounded off so you don't risk detaching the airflow.

As far as making a boat tail, it depends on your quality/appearance standards. I spent probably $500 making an aluminum framed, coroplast skinned tail for the Anal Probe. 3-Wheeler built an OEM-looking Insight tail you'll want to check out. Also peruse threads by Jethro Bodine (Mk2 Jetta), basjoos (Aero Civic EG) and Weather Spotter (Matrix).

Seems a blue foam buck covered in foam is a good approach. You can put a lexan rear window on if you use simple curvature over that area.

Make sure you figure out a good way to transition from the belly pan (you'll need one) to the bottom of the boat tail. My Civic hatch has a convenient rear tow hook for possible mounting; perhaps your sedan does as well.

Anyway, read some threads and see how they planned it. There's a ton of information to absorb.