Here's a pic in a Giulia Sedan. Same will work in a GT. If you have a smaller bottle, other good locations in a LHD car are the right side of the passenger footwell or behind the passenger seat in the rear passenger area - the latter works well if a roll-over hoop is installed and you can attach the bracket vertically with hose clamps.
I haven't got a picture but I'll try to explain what i have done.
It's quite simple actually.
The fire extinguisher is a 2kg dry powder type.
It is located behind the passenger seat, in parallel with the seat's rear side.(Vertical to the car). I have managed to stabilise it there by using one rubber cord with hooks. One hook is latched on the metal net somewhere under the seat. Then I wrapped it around the fire extinguisher as many times as it needs to leave enough cord to latch the second hook again under the seat without being loose.
It doesn't seem to cause trouble to the rear passengers, and the seat can be moved front or rear but with a bit of caution.
In my case I hardly ever get passengers in the back and I don't move the passenger seat front or rear very often.
Hope this helps.