looking for sport start clock designs

I have an intermediate knowledge of electronics. I want to design and build a start clock for orienteering events for our local orienteering club. In orienteering the competitors start the race at either 1 or 2 minute intervals, with a start clock which sounds the last 5 seconds of start interval with beeps, the last second (on the minute) being a longer beep.

Other design requirements are:

- low current consumption

- portable and small (6"x4"x2" max???)

- large LCD readout of the seconds and minutes (5 digits, 3 for minutes,

- precise quartz timing

- rugged and weatherproof

I wonder if anyone can point me to reasonably similar start clock designs which I can adapt to my requirements rather than me starting from scratch.

Any assistance greatly appreciated.