I actually have a control box I've built (but not yet wired up) using one of the eBay aquarium controllers so prevalent on these forums. Once I get it wired up, I'll let you know. Even if the probe end doesn't fit through the drip tube, however, it will be easy to just slip the bare wire end through and then clamp it into the box afterwards, so I don't foresee the need to drill further holes.
Note that it's also possible to drill out the drainage hole to widen it--I know I read about someone else doing that.
Note that it's also possible to drill out the drainage hole to widen it--I know I read about someone else doing that.