How do I make a cube rigidbody pushable by the default 3rd person character controller without tilting and going left and right?

Hi

I have a simple cube with rigidbody on and I'm using the default 3rd person character controller. How do I make it so when my character pushes the cube from its back it goes forwards only, no matter which side of the back the character is located on? Currently when he pushes it if hes on the left side the cube will go right and vice versa - I just want it to go straight always