3 questions about box-building

I'm building a box for an Alpine Type R 12, not yet sure if going sealed or ported.

1. How do I calculate the volume of internal braces in a box?          

2.  Does it matter if my sub faces up, or faces the rear trunk wall?

3.  Should I use sound dampening inside the box.  Polyfill, or dynamat etc.

1. go here https://www.the12volt.com/caraudio/boxcalcs.asp

2. Depends on the car

3. if you need it...... dynamat is useless inside the box.... use it for the car

Concerning what direction the sub faces, I drive a Nissan Altima w a trunk.  Seats don't fold, but w/ the armrest down I have a 6 x 12 opening.  I'm very familiar w the volume calcs on this site, but it only calculates interior  volume, I need to know exterior volume of the internal braces.  The box is going to be square or rectangle, what determines the need for polyfill or other dampening material?

L x W x H = cu.ft.

12" x 12" x 12" = 1 cubic foot = 1728 cu.inches

A brace that is made out of 3/4" mdf may look like this

12 x .75 x 8 = 72 cu. inches or .042 cu.ft.

If you have built your box to the internal specs as indicated, you will need to modify either the L,W or H to add the space taken up by the brace. Thus if you needed and calculated out 2.0 cu.ft, you will now have to change one value to get 2.042 cu ft. Now if you are an anal box builder (like some of us) or a die hard competitor then this will matter. If this is a sealed box, it would more than likely not make a difference at all. You can be out by about 1/4 to 1/3 total volume before it will begin to affect the performance of a sealed box.