when you are building closed-back guitar cabs it isn't really important about the internal volume...
If you play with "speaker cab design" software you will find that for an optimum cab volume it tells you to build one the size of an oil tanker or something lol...
What you want to watch out for it building dimensions using even numbers... this MAY act to increase bad resonance and dampen frequencies so the only thing to remember is always use odd numbers, ideally use prime numbers and this will not be a problem!
Odd numbers = no problem
