Well you could always buy a pre made box, but if you go that route, i would do a sealed box. If you want a ported box, have that custom built for you, to match the sub, because the ported box has to be designed (airspace, port diameter and length) properly to sound right. A sealed box is more "forgiving" when it comes to airspace the sub needs. Generally, a ported box is said to be 3 decibels louder than a sealed, and you said you want spl, so i would go that route. A custom built ported box might run anywhere from 85-175 bucks, but well worth it. You might even be able to find a premade ported box that you can cut the port to the needed length...Which model exactly are you looking at? Check out this link http://www.alpine1.com/html/D2_n_1_n_n.html
It gives a sealed box volume, but i do have a box building program on my computer if you need some ported box specs. Let me know..I'll have to check if my program has the specs for this sub in it..Opps, well it looks like the site has the Thiele-Small parameters in it, so i could plug in those specs in my program to give you a ported box size if you like.

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