Is there a way to weatherproof a cheap ($15-25) ultrasonic sensor for moderate outdoor use? I know Maxbotix sells weatherproof sensors, but these run for $100 each, which is too steep for my project. I just need a sensor to resist light rain. Is there any simple modification I can do, like cover the sensor in saran-wrap, to protect it from small amounts of moisture? i.e. What firm materials are transparent to ultrasound?
First of all, the module you linked to can be found at ebay for ~$2 (check this one) and here you can find only the waterproof sensors (Rx and TX) for about ~$5, so maybe you can get those two (the cheap module and the waterproof sensors) and swap the non-waterproof with the waterproof if their specs matches.
Or get a waterproof one that is ready with the module for about ~$14 here
or this one which is mostly used for car parking assist kits for ~$16 here
you still need to protect the circuit board but that should be easy.
A firm material by definition can't be transparent to ultrasound, you can try putting it in a pipe and sloping it downwards to avoid the rain. Or maybe see if putting some plastic wrap over the end of the pipe will allow some of it to get through, I'm skeptical though.