I've seen sites where you can buy either small .5-.8 ft^3, mid .9-1.2ft^3, and then large, whatever they have. If the box is made of 3/4" or better MDF, I'd say get the box that overshoots your needs volume wise, and buy some extra MDF and start adding to bring it down to your required volume. Heck, make some bracing to subtract volume for more strength.
I made a box custom for my trunk, and I'm pretty sure it ended up too small cuz the sub sounds horrid. I think I'm gonna save and get something custom made from audio integrations.