Ok, here goes . . .
Yes it's going to be louder. Drop the .12 for simplicity's sake. It's not going to make a difference. When you cut the pieces, be sure to account for overlap. The way I did it was to first put together the sides and front and back. Then I put the top and bottom inside that figure. As far as putting the pieces together, I used wood glue and wood screws (you can use dry wall screws also . . . it'll make it flush with the box) every 2 inches. I also bought silicone sealant and i sealed every seam of that box. trust me, you'll want to do this (especially since it's your first box. there WILL be mistakes, trust me). I was suggested 2 ports if you were going to do separate chambers, however a lot of people on this board reccomend a common chamber. If you're going to do a common chamber, then you could still do two ports, but for simplicity's sake you should do one port, 3x8, with a length of 13.45 inches. put it where you will, air will still escape it, and that's the whole point. i usually put mine in the (ok, say you're facing the front of the box where the sub(s) is/are) right rear corner, and i make it in an L shape so that the end of the port is facing towards the sub. if you do that, in order to measure the port length you just measure down the middle of the port, not the sides. As far as bracing goes, anywhere and everywhere! I've never heard of a box that was too well braced. A good idea would be to take some extra fiberboard and make right angle triangles, with each side having a length of an inch or more. Do everything you can everywhere you can, cuz the box is going to take a beating. Any more questions? haha . . . bring 'em on if you have them.
Chris