All of them. If we just widen the baffle, S1-S4 will all increase, and the flare rate changes, like GPM explained here
This messes up the "tuning" of the horn to the driver. I am not talking about phase or corner frequency, I am talking about adjusting the acoustic loading and reactance of the horn to match the characteristics of the driver. Just randomly selecting a value is a bad idea here. You've got to make sure the number you pick is appropriate for the driver.
So - I did.
I tried widths from 8.5" to 14", 11" looks pretty good. I'm sure I could tighten this up a touch more if I spent more time on it.
I suppose, if I were gonna do something like this right now, with no further work, I'd use an 11-inch width for the interior baffles, and I'd make sure the cutout was the right size for the Apex 8 and move it just a touch so that it's in the right place (edge of hole nearly touching the throat end). Other than that, I'd build it just like the pictures, and I'd use good ply, cause we're right at the edge of getting away without using any bracing.
Is it perfect? No. Is is pretty good? Yeah, I'd think so. 100 watts = 110 dB, groundplane.....
Not bad for less than 100 liters.