I keep 3-4 timers at our Dallas Club. If our local matches start to get too long -- a 20-person match should not take more than 2-1/2 to 3 hours -- I will put them out on every third lane, and ask participants to use them, just to make them aware of how much time they are taking. So it's useful to reign in some folks who like to talk more than shoot, or are just not aware.
There are a number of "interval timer" apps for smart phones. I have no idea if they can be adapted to field target. But, if you already have the phone, the price of the app is certainly affordable.
Under WFTF rules, smart phones and pads are not allowed, but should be OK to use in the USA.