Zip Lock bag and tape or rubber band works for me. It keeps the metal detector clean and dry if water just splashed on it or if you are making a dash back to the car when it starts raining.
I put a new one on as needed or when I change batteries.
I recycle the ones I use around the house. Shake a little soapy water in them, let them dry out, and they are good enough to use on the metal detector.