You always want to go with a sealed cabinet. You limit your low end reach by using a port. The trick is to lower the Q and control the driver by using a digital crossover with EQ capability Then you can make the driver do anything you want as long as you have the power to do it. I would make an enclosure around 6 cubic feet plus the volume of the driver. The problem is building an enclosure that does not resonate. Most enclosures are musical instruments. Large flat sides will resonate the most. With bracing, you just raise the frequency. The stiffest structure is a sphere, but a sphere is very difficult to work with. Next is a cylinder. You can easily place a driver in the end of a cylinder. You can get 20 - 24" pipe in any number of materials, the best would be aluminum. With a driver that large you want an enclosure that weighs at least 200 lb or you could place a driver in both ends and make a balanced force subwoofer. I made the cylinder with a decadron cross section using 1.5" plywood placing a 12" driver in both ends. 18" drivers would require a huge enclosure, double the volume of a single driver enclosure. My guess is that would be impractical. How you make the enclosure also depends on the shop capability you have available.
Good luck. If you have any questions message me. Mike