We offer a machined bracket and switch assembly from pacbrake, you can see it on their web sight...
I've had 3 customers get a switch from Radio Shack and they have drilled a hole in the shifter lever and installed it... . 2 of the 3 later removed it... always was in the way when they shifted... the 3rd guy drives 60K a year, and has replaced it every 12-14 months... he carries a spare... says they aren't made well...
It seems to me that a switch mounted on the side of the knob might be better. . but I'm used to using the bracket and switch from Pacbrake... we have it on all of my trucks... .
Hope this helps. .