Inconsistent 2D physics between operating systems

I have a game using the 2D physics systems. Put simply, you shoot a marble, it bounces on elastic bands that you’ve placed, and you hit a target. (If you want to try it yourself, the first version is online here.)

Here’s the problem: I’ve been noticing slight inconsistencies in the physics between Macs and PCs. There are times when the marble consistently lands in one location on a PC, and a slightly different location on a Mac. This, obviously, could become a problem if I want a puzzle to work in a certain way, especially if it’s a very fine-tuned puzzle.

I’m aware of putting everything physics-related in FixedUpdate, but is there any other advice that will help maintain the same results between platforms? Is it even possible?

Maybe multiplying physics operations with Time.deltaTime,
I don’t know if this is needed when using MonoBehaviour.FixedUpdate()
That depends on if FixedUpdate() is called by unity n times per frame or n times per second

For example when applying force:
rigidbody.AddForce(Vector3.up * Time.deltaTime);