Tim
I found a 4" -> 3" reducer that slides (tightly) into the 4" cap. Had to do a very small amount of scraping of an inner ring so the 20oz tank wouldn't catch but that only took a minute.
Mounted the cap to the bench, just slide the adapter in part way and I've got a 'monster cup holder' to put my regulator right where it's handy. It also means that the tank is out of the way and can't tip over.
Neil
You just need two 20oz tanks (active and spare) and a larger tank back at the hotel (or other home base) to fill them from. Depending on where and when the show is you could even fall back to getting them filled somewhere local but then you
have to get there.
At that point in the future where I do a show I'll take my Box O' Happiness, my regular ball vise, and my OptiVisor(s). The micro ball and smaller visor I keep in the Box are ok, but not as good as the larger / heavier ones. Oh - and my spare tank, of course - the Box just has the one in it because lunch is only an hour.