For automatic attack, the way I've seen it on most timers is a rotating cam with a switch on it. as the cam rotates, a switch that's fixed in place will be depressed by the cam starting up the siren. when the cam completes one rotation, there's a drop and the switch gets deactivated while the surface of the cam rises again to meet and activate the switch.. All you'll really need is a 3D printer, some switches, a motor to run the cam, and some skill with a CAD program. (Bonus: Greensand/Petrobond, scrap aluminium and a furnace).
The cam should somewhat like this:
For alert, of course, you'll just need a switch
Proud owner of a Lion King LK-100 hand crank siren. I also can also brag about riding behind a "Climax" locomotive...almost 10 years later.
Greetings from Morgantown West Virginia, home of the longest and oldest PRT system in the world!