The Balance Tank is a classic Ninja Warrior/Sasuke obstacle that's existed since 2002. Even though neither Sasuke or American Ninja Warrior uses the obstacle anymore, it's still commonly seen in ninja gyms around the world. Many of these gyms use big spools used for electrical cabling.
It just so happens that I work for an electrical company. And they were literally giving these spools away! How could I say no to a deal like that?