Eclipse Aluminum Displacement/Enclosure ?

I am planning on building and enclosure for 2 12" eclipse aluminum subwoofers, the single voice coil models.  They will be in two separate chambers firing forward in my 97 4 door civic.  The front of the enclosure will be at the same angle as the seat backs are in the trunk.  I plan on angling the back of the enclosure also and mounting the alpine m1000 amp I plan on using to power them, as well as a 4-channel amp there.  How much space should I leave between the seat back and the subs?  Just enough so that the subs will not hit the seat at full excursion?  Also I was reading the "Car audio Cook Book" the other day and it says that enclosure shape does not matter, that only volume matters and it doesn't matter what shape it's in, as long as you leave a couple inches behind subs that have vented pole pieces so that air can flow through the magnet.   This was completely contradictory to everything else that I have read about enclosure design and transient waves.  I assume that this was just the opinion of the writer.  Anyways, I plan on having the front of the enclosure angled slightly to align with the seat backs of the back seat, and I will have the back of the enclosure angled also, I think thats called a trapazoid if you look at it from the side.  The top off the enclosure will be smaller than the bottom.  I am going to make the enclosure out of 1" thick mdf, with the baffle double thick.  Oh and it will be a sealed enclosure probably 1.25' cubic.  Should this volume be with or without the sub displacement?  I'll appreciate any suggestions and comments about enclosure design. 

Also, does anyone know the displacement of these subs, or the hole cutout size and outer diameter?  There the ones that handle 500 watts rms and the model number starts with an 8.  Thanks. 

I forgot to mention that I want sound quality, and that I will pretty much alway have the seats folded down.  Would the subs facing the rear still be my best bet?  Also how large of a difference would it make, because the way I want it to look I would prefer the subs firing forward. 

Folded down seats =  much improved sound. Most subs do sound better firing towards the rear of the vehicle, try it both ways and see which one performs better. Build yourself a test box to find out.