Mine is in the trunk between the battery and the ground. Bought a high quality switch from Watson's. Under lock and key to avoid anyone getting to it, if they break into the trunk, they're going to get the car no matter what.
Of course, its not much to look at now anyway.
For the folks who put their cut off switch outside, they usually race and switchs have to be outside and clearly marked for corner workers in case of an accidient. There is another thread on CC about where cutoff switchs are, pros and cons, do a search and you should find it. Best close at hand where you can reach it in case you are upside down, along with a fire extinquisher. HOpefully that will never happen to any of us.