To answer the original question, think of an IB as a "boxless" loudspeaker. The "enclosure" basically is nothing more than something to hold the driver in place, and block immediate sound waves from the rear of the driver so they don't cancel with the front. For this reason, the bigger the box (or baffle), the better.
To determine the best parameters for an IB speaker, it's helpful to look at what a sealed box does for / against the response. Putting an enclosure on a driver raises the Q of the system. The smaller the box, the higher the Q. This has the effect of raising the output response at F3 (a good thing) but raising it and causing a steeper rolloff below that (a bad thing). An ideal speaker response will have a Q of around 0.707 (butterworth response). Since the enclosure raises the Q, and hopefully approaches 0.707, the closer to that Q you can get with the driver alone, the better for an IB system.
Also, since the idea is to get deep bass, a low Fc is helpful. The most efficient way to get that is to have a large Vas (equivalent volume); that means a soft suspension. Since it will have to move a LOT to get deep bass, a long Xmax is needed, as is a large Sd (diameter), or multiple drivers (or both). Unfortunately, a long Xmax and a large Vas is a recipe for voice coil rub, so it's difficult to find that combination. Go for a large Vas (and correspondingly low Fc), and large piston over large Xmax.
Oddly, these parameters are the same as a sealed box driver, except for the Q. The motor will reduce the Q, so a strong motor is antiproductive for a good response in an IB system, where it is a boon in a sealed box system; so yes, it pays to get drivers made specifically for IB systems. They are rare, so will cost a pretty penny. Good luck!
