I found a neat old National AB-235 solid state radio at a swap meet this spring for $5. Since it wasn't working anymore, I thought it'd make a great bluetooth radio for the beach.
To keep the original speaker and all of its tinny charm, I needed more depth for electronics. My solution was to create a new back piece with my 3D printer that was about .325" deeper. I added through holes for the electronics, a 9V battery holder, and an opening for a switch.
Here's the receiver I used