The cycle computer sensor is mounted on the circumference of a freely spinning plate (in my case a peanut butter jar lid screwed to a sliding door castor) which is surrounded by three rounded cups (baby food cups, NZ$4 the set) and mounted on a pole (an aluminium towel rail). All parts are either naturally weather-resistant (e.g. plastic or aluminium) or made so (e.g. varnished the wood, used silicone sealant to prevent water getting in). The unit was assembled and then calibrated out the window of a moving car. Thanks Chris! Link.
DIY Win Speed Meter