I have installed a small water resistant Oontz bluetooth speaker in my frunk area. It's tapped into the 12V battery directly with a fused link, so it never shuts off. (It is only rated for 1A draw when it is charging, so probably uses far less when it is not.)
Inside the car, I have a cheap older phone with a custom soundboard app on it that stays in the car and plugged in under the dash. I can load any sound I want that way, either as one-shot, only while held, or on continuous loops. I have sounds from the drones in Oblivion, Darth Vader's theme, and of course Jaws among others at the ready.
Most of them are far less alarming than the horn to clueless people wandering unaware down the aisles of parking lots.