Which is more or less what a chess clock does except you start with a full bank.If you have an iPhone or Android there are plenty of free chess clock apps that can be installed on a smart phone or tablet.
I'm in favor of Shot Clock, but think it should include a time bank. Every player starts off with a 30/45/60 second shot clock, and any unused time can go as credit towards a time bank. When a player occasionally needs extra time to concentrate on a hard shot, the shot clock immediately starts drawing on the time bank.
The clock for 3-cushion used to be that you started with a certain amount of time and every time you shot you would get 40 more seconds or some such added (IIRC). They have gone to a plain 50-second clock (I think) with five seconds of beeping at the end to warn you and some extensions. There are some players who do not start their final strokes until they hear the beeping. Brutal.