Build a bracket to hold the button somewhere within the drivers reach but in a location it isn't likely to get bumped by accident while moving(bad things can happen inside trans if that happens). I like to use a position at the dash at arms length in front of you, like above the heater controls for example. Reason being is from reading a test on reflexes, you jerk your whole arm away from touching something(like fingers on a hot burner) faster than you move just your thumb or finger...test yourself
. You'll see a lot of winning bracket racers set up the same way - on the dash, on the roll bar above the windshield, on the windshield post, even seen one sneaky in the cigar lighter hole behind the ashtray door.
Wire the button to trigger through a 30 amp relay(like a fog or driving light relay), the transbrake pulls a lot of amps, too many for a simple momentary contact button to live with for long.