New 12's. Should I modify old box or get a new one built? Pics incl.

[DaGoat]

So I'm buying 2 new p3d4-12's to replace my very old subs, they arrive here on Wednesday. 

Here are the specs for the speaker: http://www.rockfordfosgate.com/products/details/p3d4-12

Now I want to go with a ported design and want to get as much out of it as I can. Going to wire the dual 12's 4ohms to 1 ohm with an 1000rms @ 1ohm amp, but I do plan on upgrading the amp next month. I mainly listen to Rap and Reggaeton(spanish rap). I'm pretty good on my loud speakers and just want some really good bass now. 

So the guy at the shop said he can turn my current box into a ported one, just give him the dims I want. The current box I have is sealed and slanted with a 38.5 W x 10.5 D x 17.5 H (14.5 H in the rear). Now I don't mind getting him to make me a whole new box if it is going to be worth it. My trunk is a bit weird when making the box as you can see the ends on the side are a bit lifted. The trunk is 28 D x 43 W x 15 H (21 H in the middle). So yea if someone could help me with either modifying my current box or giving me specs to get a new box built I would really appreciate it ! Thanks.

http://imgur.com/a/6StBa In case pics below don't show.

[Triticum Agricolam]

Build a whole new box. Ported boxes need to be bigger than sealed boxes and the port will take up some internal volume as well.  Converting a sealed box will almost certainly result in a undersized ported box. 

[DaGoat]

8 minutes ago, Triticum Agricolam said:

Build a whole new box. Ported boxes need to be bigger than sealed boxes and the port will take up some internal volume as well.  Converting a sealed box will almost certainly result in a undersized ported box. 

Yea I figured this would be the case. My brother said it wouldn't be too big of a deal but I want to do it right. Could you possibly help with a good box design and dimensions so I could give it to the guy at the shop and get it built? I don't know much about boxes and would really appreciate it. 

[slowfkncar]

10 hours ago, DaGoat said:

Yea I figured this would be the case. My brother said it wouldn't be too big of a deal but I want to do it right. Could you possibly help with a good box design and dimensions so I could give it to the guy at the shop and get it built? I don't know much about boxes and would really appreciate it. 

Can you get away with 14 1/2 x 38 x 19 (HxWxD) ? 

[DaGoat]

1 hour ago, slowfkncar said:

Can you get away with 14 1/2 x 38 x 19 (HxWxD) ? 

Yup, those dims will fit in my trunk.