Go to Dollarama and buy a digital egg timer for $1. Last year I was working on getting this thing to trigger a personal alarm (also for $1 same store, 110dB of annoying noise) in the attempt to make a satchel charge simulator for a game I had planned. Got it working on the protoboard just fine, but the circuit I made didn't work, and I haven't looked at it since due to busy schedule.
Basically, the timer switches the ground that goes to the buzzer. Remove the buzzer, solder to and run two wires out the back of the timer. The circuit I threw together uses an NPN transistor as a switch to kick in the ground of the coil for an IC relay powered by a 9V battery. The personal alarm I took out the ground wire to the circuit off the battery and ran it outside of the alarm. Relay closes, power is supplied to the alarm and it screams. Because the timer switches in pulses, the alarm would sound in pulses, so I just used a high value electrolytic cap to fill in the pulse gaps because it charges and discharges keeping the voltage level around the same level (same way filter caps work in electonic equipment that change AC into DC.)
Here's the rough schematic I made, not sure if I refined it or not, but at least it worked on the protoboard:
Oh ya, the ARM button was meant to give the cap an initial charge. The timer only puts out about 1VDC as well.