I've gone to a professional car-audio installer and he calculated me a bassband - box for the speaker I bought there without paying something extra.

There are several programs out on the net for calculating the sizes of hifi-speakers you could look there.
You have to know some parameters of the speaker you are using then you can calculate a box for yourself.

Or you go to a good hifi-studio and talk to some people - maybe someone is known in calculating speakers. You'll get a good sound out of custom-boxes

TeeMcBee