That depends on what you mean by "impromptu" in this case. Do you mean propless, or just able to do any time?
Because if you carry a notepad and a pen you can do billet work for days. No gimmicks needed. Center tears, peeks, switches, so on.
If you want to go propless, there's Ouija by Fraser Parker and Ross Taylor. I know there are others, but it's not a field I'm really in to so I can't think of any other examples right now. Fraser Parker, Ross Taylor, Matt Melo, and Peter Turner are most of the big names. Ben Seward, too, I think.
Personally I find the propless route often ends up being so process heavy that it loses all theatrical value. I mean, you can peek a billet in 1/10th a second and make it psychologically invisible, or you can replace that with 10 minutes of pumping for individual letters of a word or name. I'd rather get the work over fast and focus on the theater of it.