I did an rapid fire mod to my battered Logitech MX500 mouse. There are many howtos around, i used this one: www.instructables.com/id/Add_a_rapid_fire_button_to_your_mouse_using_a_555_/
The difference in my approach is: I did it without an additional switch. So the mouse is not cutted and the mod cannot be noticed from the outside. It is a clean mod.
But the circuit does need a button to activate the rapid fire, i used one of the existing buttons of the mx500.
See the pictures.