Those cheap 57mm speakers used in many hobby projects don't come with mounting holes. This design fixes that. It's customizable for any speaker size.
The speaker shown in the picture stuck out a bit, so the holder won't be flush with whatever it's screwed in to. You can live with that or adjust the depth to suit your speaker. It's probably a good idea to make the holder a bit smaller than it needs to be, so the speaker doesn't rattle.