Sometimes those old "doorstop alarms" appear in thrift stores and they have tiny, 3-volt sirens in them that are about 2" diameter. The sirens have 8 ports, though, so you might consider using these in a miniature T-128. Leo here is just for size comparison, but being a Maine Coon, he could probably fit it in his mouth (if he just didn't bolt whenever I sounded it!)

Lex orandi, lex credendi, lex vivendi.