Until a few years ago our best solution for a shielded computer mouse was made with several layers of self-adhesive copper foil on the inside of the mouse shell. Externally it looked like a normal plastic mouse. The results it terms of shielding were very good, the pain in the palm and the fingers had disappeared.
However the modified mouse had still some problems: there were two small holes for the left and right button, and a big hole for the middle wheel, that couldn't be covered with the metal tape, and so some electromagnetic field or radiation escaped from the holes. The user's index finger is often right above these spots so there are still negative effects. Also the construction of this shielded mouse with adhesive copper tape was extremely difficult.
In 2015 we found another solution that reduces the electromagnetic fields of the computer mouse, see optical USB cable.