OK, I took a peek at things and worked with the drawing and model so that I could calculate the effects of the throat end errors due to a centered driver cutout.
If you've built one of these and you centered the driver cutout on the baffle, you didn't build what I did.
The baffle in the one I built looks like this:
Sorry that this wasn't conveyed more clearly in the instructions, it is exactly what was drawn in the sketchup file though.
If you've built these and you centered the cutout on the baffle, that's OK. As it turns out, the fix is easy, and the differences between the fixed version and what I built are minimal.
To "fix" it, you can just add a ~3/4" thick strip to fill in the throat end of the horn. If you cut the strip in half, the pieces can be inserted through the driver cutout, and a couple screws and some PL will hold things in place.
The strip should be about 1-13/16" wide, although you might have to bevel it a touch to make it fit snug. Measure your builds to be sure. This won't quite fill the throat end, but the 1/8 inch or so remaining is not that big of a deal.
The differences may or may not be audible, but should be evident in a measurement, especially in the impulse.