Modeled after an existing one, designed in SketchUp.
Untested design, but in theory it should work. Suggestions are very welcome!
Edit: It comes out fairly well, it just needs some adjustments and fine tuning.
You'll need three AG13/LR44 batteries, a spring and a metal plate to make contact in the battery compartment and four 5x20mm springs.
You should use a 28mm speaker, but other sizes might fit.
Fix electronics to the bottom part, and the speaker into the ring in the top part.
Slide the springs onto the pegs inside the bottom part.
Close the box by pushing it together
Insert the batteries and close the battery compartment with the lid.