You made a good choice getting the single 9515, IMO. However using 1/2" MDF is a waste of that good decision. Having the 1/2" on the top, bottom, sides... doesn't matter, it will flex like nobodies business! If you can only get the 17" or for some other reason you can't use 3/4", I would highly suggest when you cut the 1/2" for the bottom or top, you then cut another 1/2" board that will fit on the inside of the side walls so that you are doubling up on the 1/2". Spread glued over the entire area, and screw it together with 7/8" screws. You will essencially end up with a 1" bottom, but not increase your total height. I would also suggest getting some 1" dowel and bracing running a couple vertically and one horizontally.
If you have a router, round over the port mouths (interior and exterior) and the port corner, put 45 degree bracing on all corners, this will help air flow and strengthen the box. Use the hell out of the glue!
the strength of the box is just as important as buying a good sub! You spent good money on a good sub, now do it justice by building a box that can handle it.
I hope I didn't offend, just my 0.02
Good luck