I'd go with the recommended boxes, there's not much to fiddle around outside the recommended designs.
• Sealed (home theater/music): 4 cubic ft. (net internal, not including driver) for an f3 of 31 Hz, f10 of 18 Hz with a Qtc of 0.72
• Sealed (music): 7 cubic ft. (net internal, not including driver) for an f3 of 31 Hz, f10 of 15.8 Hz with a Qtc of 0.62
• Vented (home theater): 8 cubic ft. (net internal, not including driver or vent) tuned to 20 Hz, with 3 flared vents 4" diameter by 28.5" long, for an f3 of 18 Hz
• Vented (infra-sub): 12 cubic ft. (net internal, not including driver or vent) tuned to 15.5 Hz, with 3 flared vents 4" diameter by 32" long, for an f3 of 15 Hz
If you got enough power you could also use a linkwitz transform to make the low end response better in a sealed alignment.
Could build a marty box that's what 95% of people do with them. Full Marty's are huge but will give you some great lows and shake the house down. Look up marty box blueprints theirs a few different sizes.
Overlord1330, first this is a forum, it is best if you don't use the PM for anything except things that are truly private. Sharing questions and answers makes it so others can gain wisdom from the collective knowledge of the forum.
Your PM to me was;
I got a crown xls 1502 to power it and box design no problem any size will work I want it ported. I know that somthing tuned to mabey 18hz
I haven't modeled UpKing's recommendations, I will later. When I used WinISD to define optimal box size for a ported enclosure the size was pretty big (21 cu. ft.). Which is why I said what I did about it being tough to design for. One thing that will help any sub system is a DSP. The crown xls 1502 has a DSP built in which will allow you to tweak the parameters to compensate for suboptimal box & room conditions. You could either drive one voice coil per channel or bridge the amp and connect the two voice coils in the driver in series (4 ohms). The results should be similar.
You haven't defined what the purpose of the sub is. Some choices could be;
Nor have you specified the size of the area, small room, large room, concert hall, open field?
As UpKing implied, different uses dictate different designs.
Fill in the blanks and more people may step in with their experience and suggestions.
I assume you are running the 4 Cu Ft sealed PE box. As UpKing noted you will only get an F3 of about 30Hz, not great (without help) for HT.
A 14' x 20' room assuming 8'-9' ceilings certainly should be able to be energized by this woofer. If you are using the Crown amp in bridged mode and the woofer in series mode you should be getting a maximum of ~1,550 watts. In the sealed box you have you will need to apply a boost of about 10 DB at 22Hz to flatten the curve enough get decent HT wooferage. Your DSP should allow you to make this adjustment, I would think. Although I am more familiar with a parametric equalizer.
One thing that most women will tell you is that placement is as important as size. Have you done a woofer crawl?
Here is a simplistic explanation, but I would recommend some form of DB meter (you can download an app for your phone) and a subwoofer pink noise generator.
I'm not sure where you are located, but I live in Grand Rapids, MI and I am trying to get rid of a full Marty box made for an Ultimax 15. it is around 11.5 cubic feet and tuned to 18Hz. It is designed to look like a decorative chest. PM me if you are interested.