Hi Thomas
As an external speaker, I can also recommend the one that Al recommends here. Makes a good sound.
In my original DGTPi I built this active loudspeaker internally, without a housing and without a battery.
If you are toying with the idea of doing that too, first think about where you get the 5V for the loudspeaker board from and how you connect the signal input of the loudspeaker board to the 3.5mm jack socket. Since the socket can only be reached from the outside, I soldered the 3 wires directly to the socket on the underside of the Raspberry and connected them to the loudspeaker board.
(not an elegant solution)
I turned the Pi on my substructure, so the jack socket comes in and soldering on the Raspberry is superfluous. Still, I use a sound card with this speaker.
greetings