I use a darkroom enlarger timer. It has a large face with a sweep second hand.

For added effect, I put a red 25 watt party light in a plug-in light bulb socket (a lamp socket with electrical prongs instead of a cord) and plug it into the top of timer. The light goes out, the speaker shuts up... at least in theory.

This set-up seems to encourage the presenters to condense their ideas and proposals. It also works very well with folks who drop into my office saying, "I just need 5 minutes". I set the timer, start it and say "Fine. When the light goes out, you go out." You'd be amazed at how concisely a vendor can pitch a product or service when they're under this metaphorical gun. heh.