A good place to start would be building a simple buzzer based on projects you can find online and then building upon your knowledge gained from the project in order to build a more complex horn. Once you reach that point, or if you already understand the mechanics of the device, start by making a plan of how the parts will fit together, how the magnet and components will be mounted, etc. A 4" conduit box extender ring would make a good frame for the horn, and then you could cut a piece of flexible metal to size and mount it to the “back”, where the extender ring would normally connect to the main conduit box, using the existing mounting holes and some screws and nuts, for use as the diagram. Mount a 2 gang adapter ring over this, to keep the plate in place and allow it to vibrate correctly. Then you could use a blank plate, which would attach to the “front” of the box, to mount the electromagnet and switching mechanism. Finally, using the two gang adapter plate added earlier, you could mount a grille or cover over the diaphragm, or in the future, even a light or strobe assembly. If you want I could make a diagram of how I envision this going together, or maybe even a picture because I now feel motivated to build one :lol: