I'd make the box atleast 16" tall, and from your port specs, it sound like you'll have to make the box atleast 23" deep (you usually need atleast 3 or 4" from the end of the port to the back of the box). Keep in mind that you will have to take into consideration the width of the mdf board (3/4")...so work that into your dimensions. I'm not too sure about that "L" shaped port, I would guess it wouldn't work too well. You might want to consider two 10" ports vertically on each side of the sub. I'll try to draw something up for you and post it up. And yeah, you'll probably want to make the box about .2 cubic feet bigger to compensate for the sub's displacement.

*edit*
I just did some calculations and with the given port depth, it's virutally impossible to create a rectangular box much less than 4 cubic sq. ft.. Is the recommended air space they gave you a minimum? Will it work as well/better in a larger box or what?