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You can try to adjust the values in the PhysicsMaterial, and try different ways to move, like AddForce, AddVelocity, Translate and I think there’s something else as well. Don’t be afraid to try different things, you will learn a lot through trial and error.
When I made this, I just let my cube roll around, so the player will have to spend extra effort in controlling its direction, and I added a jumping function, allowing more flexibility. The result is kinda different from the tutorial, but that’s the spirit of developing game, shape and merge a lot of tutorials  into something new. Keep on!