I wasn't sure where to ask this question, but since it's actually an Arduino project, I'll try it here.
I want to upgrade my old car with ultrasonic parking sensors: I've already bought HC-SR04s as sensors and started to play on my breadboard.
But what I'm still unsure about: if I place them in my car's rear bumber, how can I make them resistent to rain or dirt or something like that? As pictured on Wikipedia, the sensor is covered with a small piece. But what could I use to cover the holes in the bumper, without interfering with the ultrasonic measuring?