Don't rule out the removable stick-on pieces that are for pictures. They work very well, and depending upon the size, can hold quite a bit of weight. This eliminates holes in the wall and possible puncture of electrical and/or water lines. We use them for almost anything we put on a wall.
Recently I added two more fire extinguishers, one under the sink and the other in the rear closet. With both, I used the stick-ons on each holder, with a small strip of velcro running down the side of the extinguisher near the bottom to keep it from moving during travel. Works great and is removable if needed.
Ron