I would like to introduce you to one of the cheapest, yet decently (for this price shelf) playing amplifiers with built-in bluetooth.

I prepared a simple enclosure with a universal connection panel for power and speaker.

The biggest advantage of this amplifier is its wide power supply voltage range, starting at 9V DC and ending at 28V DC. For this reason, all power tool batteries, car batteries, or plug-in power supplies with a capacity above 12V 2.5A will be as good as possible to power this design.

I have tested and 4 as well as 8 Ohm speakers play.

Shopping list:
1x ZK-301B amplifier
2x red connector
2x black connector 
4x insert for M2
screw 4x M2x6 [mm] screw
1 meter of 1mm / 18 AWG wire for internal connections

if you are using power from an electrical outlet then the power supply from the link will be suitable (12V 3A)

In the photos, a repositioned ABS printout