Weatherproofing a cheap ultrasonic sensor
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.