We carry two. One's mounted on the wall under the companionway...just aft of the engine box. We took out the door to the aft cabin, along with the non-supporting wall next to the engine box. Used the fire extinguisher to hide a couple of the holes that were left behind from the wall removal. With or without the door and wall removal, this could be a good place to mount your extinguisher, as it's close to the galley.
Our other extinguisher is kept in the cockpit, in the small, aft, starboard locker. If there's a fire down below (sounds like Bob Seger?!?!) while I'm in the cockpit, I don't want to have to go below to get the extinguisher.