Waterproof, inexpensive, inconspicuous, and quick to build - let's go!
Step 1: Stuff You'll Need
-A dime. Yes. A dime.
you'll also need a hot glue gun, but that's it!
*This might seem obvious, but it's generally good practice to find a sprinkler that your keys will fit inside of. If possible, open up the sprinkler you plan to purchase/use to see if your keys will fit. If you have a car with a large key fob, you might have a harder time finding the appropriate sprinkler.