I'm a fan of IB's since building mine with 12 of the the cheap 15 inch drivers that were available in Australia from Jaycar.
Since those drivers are no longer available, we've had nothing suitable for an IB until the recent arrival of the Tempest-X driver. The problem is that the new drivers give double the performance of the old ones, but are four times the price ($350aud ea!!). This makes a high displacement IB fairly expensive for us. The solution may be to use the LLT solution, as expoused by Steve, and demonstrated by Rodney.
Here's a comparison showing the Tempest-X driver used in a 4 driver IB, a 4 driver LLT and a 6 driver IB that costs an additional $700aud for the drivers. The shaded area shows the gains from the port. A pair of 8 inch ports are OK for velocity, but would be a bit short - you could always go for
a pair of 10's
The LLT might be just the shot for those on a limited budget, and with a smaller space to work with.....
WinISD project file for LLT design Page containing project files and futher details of the IB designs
viva la difference!