Proper AKABAK interface?

When measuring the dispersion of a waveguide in AKABAK, should it be modeled as an external domain with no interface, or as an internal->external domain with an interface? And if the latter, what shape of interface should be used? Both of the approaches seem to give reasonable, albeit different results. Here is the waveguide I am trying to model for reference:

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On a somewhat related note, I'm trying to get a 100x80 dispersion pattern for my waveguide. Is it possible to get +/- 50 degrees smooth all the way up to 20 kHz using a compression driver? So far I've had better results when I tried testing a dome instead of a flat driving surface.