Dual SP4 Encloser recomendations

[NFrank97]

I would like some help on some box dimensions for my two Fi SP4 15's. I have a 2000 F-150 extended cab and want to build a custom ported box to go in the rear section of the cab (removing the back seat to put in box). The total amount of space that I can work with is 24" from the back wall of the cab, 62" from door to door, and 24" tall. What dimensions should I use to build my box? And i dont want any recomendations on certain programs that I could use to determine what I need, I have tried a few and now of them are quite what I am looking for. Also, what is a good frequency to tune the box to? Thanks for the help, Nolan.