Subwoofer enclosure tuning?????HELP!!!

Hi, new to the forum so here it goes. I am going to purchase a single 18 inch FI BTL N2 subwoofer for my Mazda 3. SO the real question is, how big and how big should the port be to satisfy my subwoofers needs. FI says that the prime enclosure would be between 4.5 cubic feet and 8.0 cubic feet. I want to tune my box to 35 hertz. The only issue I have is the height in my trunk. I calculated the xmax of the sub and the height of the box can only be 18 inches. I am putting the sub on the top of the box due to the height of the trunk. If you guys could help me out that would be really sick.