Fire Extinguisher Mounting Pics

Under the passenger's seat, parallel to the driver, with handle facing rearward. That way I can grab it instantly, without leaving my seat.

I always mount ours on the floor between the seat and door frame of the drivers door. Been mounting extinguishers there for over 30 years. Have put out a few car fires with them.

I just have mine stuffed under the rear seat where it's still accessible. Figured if I mounted it anywhere, the someone'll knock it off the bracket (probably me).

In my 07 Classic I had it mounted inside the rear driver side door between the seat bottom and door. There was a flat spot in the plastic near the seat belt that worked great.
In my 13 there wasn't enough room there and I had to move it up a bit to between the back of the seat and the cab corner. It isn't nearly as easy to get to....but it is in the same area.

I don't have a fire extinguisher and I don't know what size you are mounting, but I have a big mag light that fits very well between the driver's seat and the center console. If you are using one of the small ones, you could probably fit it there.