The PCB has electrical buttons permanently attached. The 3D printed parts have protrusions that touch the buttons.
You could possibly redesign the 3D printed parts only, but at some size increase you may need to start looking into redesigning the PCB.
The good news is that the mouse itself is bigger than the picture makes it seem. You might actually be OK without modifications.
If you do want to give it a try, I highly recommend buying the kit and fully assembling it yourself. That way you know that it works, and you have a solid foundation to apply the modifications to.
