Look to the suppliers of industrial electricians. ("Yelow Pages" ?) They have the "real" "EMERGENCY STOP" type of buttons.
They have switch/actuator(the button, handle, or whatever) modules for reasonable prices ("each sold separately"), and the actuator mechanisms are built really TOUGH: big aluminum buttons for mounting in panel holes of 1"+ Diameter, for example. These are made to be operated by workers with work gloves on.
The actual switches are not a lot more expensive than the microswitches at Future/Active, and they fit with no jury rigging. The choice of momentery or "locking" is part of the actuator, while the switches are momentary single Pole NO or NC. Often, there is room for three switch modules on the actuator, although you'd only need one switch for KILL (unless you need another switch for a light in the pushbutton 'for extra money, but oh so cool'
).
The inexpensive lines are still quite rugged compared to the typical "electronics" type of switch you get at RS, and the variety of actuator types is amazing: want a kill switch you need a key to re-enable your robot ?
And, you can get little plates that install under the actuator which say things like EMERGENCY STOP, JOG RUN, and other humorous sayings.
