Can I ask why you want one? If you are having it for trackdays and road use, then I think its safe to assume you will be able to get your harness undone to reach it. In reality, its going to have to be a minor fire for you to bother. Accident plus big fire and you you will leaving the car, not fiddling with a hand held. If you want it for race, then get a plumbed in system, someone like Lifeline will do a decent system suitable for a sports car.
Having been a burning car on a race track with a plumbed in job, they are effective, but largely because the nozzles face over the fuel lines and engine. If you want to look authentic. Then a floor mounted system in the passenger well is the way to go. The NGT had twin handhelds in the passenger footwell. The black NGT on the GTClassics site shows this arrangement. Not seen similar for sale.