The dimensions are outside dims. and the plans assume a 3/4" wall thickness. So if want thicker walls you have to go out(larger) or risk changing the cabinet loading characteristics. One thing the plans don't show but Wayne has said he sometimes uses, is a cabinet base riser. I put a 1 and 1/2"high ring of left-over Baltic birch screwed/glued to the base which lifts the horn up to about seated ear level which helps with the highs,IMHO. I think if I were to start over I'd make the ring(actually a rectangle) 2" or maybe even 3" tall.This riser helps with the appearance as well- it's a common feature on cabinets of all types.

Good luck with the assembly- your in for a treat on completion!

Garland