You use a shader that has surfaceparm forcefield
in it. This can be one that comes with the game or one you make yourself such as
Make a func_usable and apply this texture to it, then whenever you trigger it, it turns on an off.
I'll be honest, I didn't test this before I posted, but going from Elite Force, this is how to do it. Maybe JA has a 10x easier way I just haven't looked yet