I've had this old metal suitcase floating around for years so I finally decided to do something with it: Build a rugged portable sound system.
I used a cheap Makita USB Adapter from eBay and a small Bluetooth Amplifier that runs on 12-24v so it was perfect for my 18V Drill Batteries. The rest is a pair of used speakers from a second-hand store. Overall cost: About 60€. The final product is pretty loud and runs for hours on one 5A Battery. You can still charge your phone via the USB ports at the same time. Very sturdy, but also pretty heavy!
Feel free to ask questions.