I've been using a Garmin eMap as my cyclometer for over 10 years now. It is also not at all waterproof. But it fits perfectly in one of the small 'snack-size' ziplock bags (about half the size of a regular sandwich bag). The handlebar mounting hardware still works fine with the GPS inside the bag - the screw holding it on pokes through the bag but is tightly sealed against the mount so there is no possibility of leakage there. The TomTom models are a little bigger so the regular sandwich bag would probably be more suitable.