I stumbled upon the Tombox, a company that refurbishes old 60 and 70ties loudspeakers, insert an amp and battery and sell them as a mobile sound system. I liked it very much and so I copied them.
I started out with an Braun L450 because I liked the shape, metal mesh front and the thinness (just over 10cm). I inserted an Kemo 18W amp module, 10k poti with switch a a 2.2A lead-acid gel battery. Feet and a handle was next. With that I was feature equivalent to the Tombox.
Then I inserted another one of the Bluetooth audio modules as used in my up-cycled headphones. Now its better. ;-)
To be fair, I did not touch the 40+ years old frequency dividers and the bluetooth module does not really play nice with the amp (some distortions, should be fixable). And its quite heavy, curtsy first to the wooden enclosure and second to the lead-acid battery. But I really like the result.