I don't know about aperiodic vs. BR for your application, but with h= 30" I wonder if you have modeled this for quarter-wave action? The tall thin box is likely to have more than Helmholtz resonance going on.
My second thought is: have you considered building a Metronome? I have built two of them: FE108eSigma and F120a. Both work quite well (although the F120a doesn't like SS amps.) Tall, thin, high WAF.
If you are looking for good software to help with modeling, I can recommend MJK's MathCAD worksheets (see
www.quarter-wave.com ). I began using them after I found out that I could download a free trial version of MathCAD (MathCAD Explorer) from Martin's site (or other places.)
Cheers, Jim