I hope it is not too late but remember to account for the volume of the internal bracing and the speaker cones and drivers when calculating the internal volume.
I'm not sure about how to deal with the volume of the ports themselves. You might have to add those in too.
These will add up to more than you might imagine and change the external dimensions by several inches.
Also, use tons of glue on the bracing or you will hear buzzing from the parts.
Good luck.
Hal