I once wired a cheap 120db siren to a 9v battery and a pressure switch with the default "on" position. Packed it in a plastic pint sized ice cream container wi

I once wired a cheap 120db siren to a 9v battery and a pressure switch with the default "on" position. Packed it in a plastic pint sized ice cream container with a hole drilled in the bottom for the switch. Packed that full of rocks for weight, with the switch pointed down towards the floor. Packed it all inside an Amazon box with a hole cutout for the switch. Lift the box, and immediate piercing sound blast. For < $10 in parts off AliExpress. Unit price could be a couple bucks max at volume.