Bent,
My dual sub is almost indentical in box dimensions as your proposed sub. Mine is 39" tall, 18" wide and is 20" deep, 1.25" thick walls with the internal bracing. I have a large 2" x 2" brace across the front of my cabinet behind the baffel and between the drivers to both secure the baffel and cabinet as well as reducing resonance. It is also internally braced. I hope to have this one finished next week.
I'm at about 5 cu ft. with 4 passive radiators. I'm lining it with 1.50" eggcrate acoustic foam. This one will be the sister sub for the one shown in my avatar. I'm excited about having two of these in my HT.
You should be very happy with your results. I chose the black "Slate" Pionite laminate finish as it matches my other sub, the mains, TV, and equipment rack. Your finish coat will increase your WAF factor if it matches your finished room. I also have elegant black snap on grills which can be seen in my signature links. Post your pics for the rest of us when you are done!
Have fun,
Mike