This bluetooth amplifier is based on a PAM8403 amplifier and a bluetooth module. (Aliexpress)

Total cost price of both is 1.80 US$, as you own already most of the other components.

My original idea is to mount it in the ceiling of my bathroom to listen to music.

Most people have phone chargers and speakers lying around. PC speakers will also do it.

This bluetooth amp is very small and will fit easily in a matchbox.

With special thanks to Tonep for inspiration and constructive ideas.

Supplies

This bluetooth amplifier is based on a PAM8403 amplifier and a bluetooth module. (Aliexpress)
Total cost of both is 1.80

One USB phone charger.

Two speakers

Electric wire

A soldering iron

Step 1: New PAM8403 With Mini-USB

Be careful to buy the NEW PAM8403 with the mini-USB charging port, and no other PAM8403.

This is because the New PAM8403 with a mini USB, is the only one that will accept the Bluetooth module you can solder to the PAM8403.

Be careful to buy the right Bluetooth receiver module, with the long pins, see picture. Cost approx 1.30US$.

Step 2: Bluetooth Audio Receiver Template

The exact name of this item is "Bluetooth audio receiver template,stereo wireless speaker power amplifier modified DIY Bluetooth module 4.0 . Cost 1.30 US$ on Aliexpress.

It is easier to trim the long pins, before soldering them to the PAM8403

Step 3: Solder

Use a fine tip solder gun. Solder the 4 wires for the speakers.

Position both modules one on top of the other. See picture.

On the bottom, the Bluetooth module, on top the PAM8403.

Use very little solder flux, as overflow is a pain.

Tip. If overflow occurs, heat the solder, and bang it on the table, the superfluous solder will drop off..

Step 4: Connect the Speakers

Most speakers will work with this small amplifier.

Connect the + and - of the amp to the + and - of the speakers.

You can use PC speakers boxes, but have to disconnect the build in amp.

Special thanks to Tonep for mentioning it

This is a 2 X 3 Watt amplifier, without a volume control.

Volume is set via the smartphone.

Step 5: Connect the USB Charger

Find a USB charger and plug it in. Any USB charger with a 5V output will do.

Other option is to use a 5V stabilized power supply.