Building box need help

[Sky]

Im wanting to build a box for one T115. I have 2 T1s but the box there in is much to small and sound terrible. So I was figuring ill just put one of them in my ext truck. Its going behind the driver side where I cut off 1/3 of the bench seat. Before I had my box standing up with 2 of those subs ands its just very impracticable. I looked about the high output size if the box and found 4.22 cubic feet. with a 3" x 16 1/2" x 22 1/2" slot port and said to have a tuning frequency of 38hz. Now isn't that a lil bit of a high tuning? I thought Id wanna tune a lil bit lower then that. Im not very knowledgeable about all this stuff so I'm trying to do my best.

Im basically just looking for a good box design I can use to put in my ext cab.