The design is like a Voigt Pipe with an extra fold that
fires out the top instead of the bottom. When placed in a
corner, the walls load and essentailly increase the
flare of the horn mouth or terminus

With the driver specs that you have the gang that's
building the BIB site at:

http://www.zillaspeak.com/bib.asp

can give you some dimensions.

One design element of that site is it's not real clear on how
to move around. Click in the red "how To Build a BIB" to see basic design principles. Click on the speaker logos for alignments and
specs for individual drivers.

Have fun.

• The BIB thread at DIY Audio