Here is a little circuit I made that will allow a momentary switch to turn on/off a load.
Q1 can be any NPN transistor, and Q2 is any N-channel MOSFET capable of handling the current being switched. It should be fine with a range of ~5-12V but I've only tested it at 7.4V.
It somewhat mimics the action of a newer console's power button. I made it so I could switch a relatively large amount of power with a small tactile switch, press for on, press again for off.