Sundown U15d2

So up until this point I have always bought a box as i have never had a system with ver 1000rms and I tried using the wizard with the thiele parameters and I get to the final step and it says warning about port length being to long or something halp! Lol

Thanks in advance, whole Box designs and suggestions greatly suggested as I have a blank slate minus vehicle an speaker selected I have yet to settle on An amp due to my crazy amount of reviewing and comparing I do thanks I also have all tools minus router table to do any cool designs suggested

Edit: I have a early 90’s rodeo so plenty of space in rear

So I have purchased the amp and speaker but now I’m curious if this speaker choice is maybe. It so good, I have the sundown u15 dual 2 ohm and I bought the down4sound 2300watt monoblock the amp is impressive but I am still at odds with the box design sundowns website says 3cu ft for volume and port tuning of 34hz and that box seems small for a subwoofer as capable as the u15 appears specs wise, any advice and pointers would be appreciated thanks guys