1. It doesn't have to be L shaped, just as long as the port entrance is 4 inches away from any wall.
2. There's an Enclosure Calculators here:
It will give you the panel sizes and you can figure out how to best place them on a 4' x 8' sheet of MDF.
3. With this setup, since the amp has no Hi-pass filter, the sub will start to bottom out below 16 hz if pushed hard. If you were to get an E.D. eQ.2 and add a HPF at 18 hz, the air speed would drop low enough that you could use a 6" PVC port 17 inches long and also protect your subs from over excursion. Without a HPF the air speed would be 35 m/s which is too high.
4. Go with 9.0 ft3 net volume. Which port volume are you referring to, the slot port or the 6" PVC?