I'm studying a few of the Voigt designs and want to attempt
it in a bit. Over at the melhuish site (now called
www.fullrangedriver.com) TC has a project called Bigger Is
Better using the Voigt principle as well.

What I am scratching my head about is whether to use
the straight side or the angled side for driver mounting.

There is considerable writing on having slant fronts
on speaker cabs these days. Plus there are a few companies
using the slant front idea: Usher and Swans come to mind.

So my idea is to make a hybrid slant front Voigt pipe.

Is there any contrary information on this that I've missed?

Great job. :-)