Long sous vide cook times means evaporation, and low water levels can ruin your meal. You can retain all that moisture with this simple, reusable trick.
If you're about to start an extended sous vide cooking session, J. Kenji López-Alt at Serious Eats suggests you top off the bath with ping pong balls. The ping pong balls act as an insulator, trapping heat and moisture in the pot. They're cheap and can be reused every time you cook, and you can store them right in the plastic container or pot you sous vide in.
You can achieve a similar effect with plastic wrap but, as Alt explains, this method is better because steam isn't forced out of a single hole right by your circulator. Plastic wrap is also not reusable, and it makes adding things to or retrieving them from the pot a total pain. With this technique you can cook as much as you like and easily add things as you go along.
Why You Should Add Ping-Pong Balls to Your Sous Vide Bath [Serious Eats]