[squarefoot]

There are many projects of bluetooth amplified boxes using class-D modules including BT receivers. Searching for "diy bluetooth speaker box" on Google Images returns some interesting results. You just need then to find the suitable amplifier without BT and stick the RPi inside the box. A word of advice though: the Raspberries have a sub par audio output which isn't good for music, so you may want to consider either a different SBC or an external USB sound card, as even the cheapest will sound a lot better than the RPi on board one.