The handlebar switch for the Shimano EP8 (STEPS SW-EM800-L) seems to break off quite easily. Since I couldn't find a replacement part, this is what I came up with. Depending on how your switch broke off, you might have to take the switch apart via the little screw inside the handlebar mount and clean up some plasic to make the main body fit.

There are two parts. The main body is supposed to be printed with a softer filament (I used TPU) and the buttons out of a more rigid plasic (PLA did the job for me). I just used some superglue to keep them together. The little "legs" of the main part should be glued into the grooves of the handlebar switch.

Enjoy :)