I have a Neptune 4 Plus, and I found out that due to vibrations, the belt tensioner could get loose over time, so I found a model for the Neptune 4 Max to prevent that problem. 

The x-axis piece fitted without any trouble but the y-axis was too large. So, I decided to change it so it would fit the Neptune 4 Plus, and added some chamfers and fillets to make it a bit nicer.

If you want the x-axis stopper, you can go to the original model.