How can I make ultrasonic sensors resistent to splashes and dirt? 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?
I don't think you'll be able to cover that sensor in a way that will still allow it to work. There are waterproof ultrasonic sensors out there, here's one on Amazon, for example.

The other thing you might want to look at is IR instead of ultrasonic. That's easier to weatherproof since the IR emitter/receiver are already covered. You'll probably have less range with IR, though.