Max-X High Excursion 15" Subwoofer for only $200 - Build Ideas
Looks great. A few thoughts. If you will be using DSP for your high pass of 18.5hz, I would change put a 4th order on it @ 16hz. The response will be practically identical, but it will protect your driver better and may even allow you to use a smaller port if you want to save some room.
My only other concern is your first port resonance is rather low. It is right around 160hz. This could be audible, even when crossing over at 80hz.