Hey, I am looking for a plan for a custom sub box for a '91 hatchback? Or at least detail pictures? Thanks..

google it.
that would be your best bet.

what are you looking to put in?

Doesn't matter really. Two 10's, or one 15...one amp..

making a fiberglass mold of the area you want to put the box and building from there is your best bet

If you can find someone to build it, such as yourself, I can design you one.