Why not use chess clocks? Paint the buttons different colours so no one will mix them up, set them to a certain time (say two hours on each clock), place them at capture points. Every hour game control comes by, records how long each team was in control, and resets the clock. As long as nobody forgets to hit the clock then it should be fine.
They might be a tad more expensive (I think $20+), but it does exactly what you want and requires almost no time to set up.