After looking at other options, I decided to mount the fire extinguisher under the port side bunk, next to the battery. Maybe this is a bad idea, if the battery explodes, but I can move it later if needed.
I used the battery strap to secure the fire extinguisher. There is room to slip the extinguisher out without loosening the strap. Notice the (2) 12V receptacles near the upper edge of the photograph. These will be handy to charge digital devices when motoring. Remember, the 25 Yamaha has an alternator for charging when motoring.