I spent several months this year mixing commentators for live sports broadcasts. We had a smallish Whisper Room (about 8x6x7 or so) with a desk and picture monitor that routinely had two commentators crammed into it. The commentators used the Sennheiser HMD25 headsets you commonly see on sports telecasts. I did not much like the sound coming from this booth which I thought was way too small for two commentators and put them very close to one another. The desk, windows and video monitor made for a lot of reflective surfaces and I think the stock interior was pretty reflective as well. In the end, we made it work but it was a continual battle for me to maintain a natural, non-boxy sound, especially given the variety of commentator styles and volumes I dealt with. The studio engineers added a lot more sound absorption to the interior and I used EQ and some expansion to control the "sounds like two guys talking in a box" problem. I can't speak to portability since it was never moved...