Fire Extinguisher Mounting Pics

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.

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 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).

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.

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