I haven't built my own box because I don't have the time to mess with it but I would think the two requirements are simple - airspace requirement and mounting depth. You can get a general idea by searching on here what is possible. 5.75" mounting depth and .5-.7 cu. ft is what is generally achievable in a stock looking box configuration. given that information, you can research subs but I like to search the archives for information. I'm sure there are tons of plans posted in the appropriate forums. the 97 through 2003 body style sold a hell of a lot and there is a lot of information posted on this site for that. I used an MTX Thunderform with two JL 8s in my 97 supercab and it sounded pretty good (for 8s). I might add that the previous truck models may have had less space than the current generation.