This is my mini sub

It uses two 2-inch speaker on the front

It has a small port that exits through the middle of the speakers

The speaker hole has a diameter of 1.85 inches in case you want to put a different speaker into the box.

The back half slides out, so you have easy access to the speakers

It is 5 ½ inches long, 2 inches in depth and 2 ½ inches tall 

Below are the links for everything I used

The parts I use did require a little bit of soldering 

Speaker- Amazon.com: Cylewet 4Pcs 2inch 8Ohm 5W Full Range Audio Speaker Stereo Woofer Loudspeaker for Arduino (Pack of 4) CYT1121 : Electronics

Amp-Amazon.com: ECSiNG 2 Pack 3.7-5V Bluetooth Receiver Module Audio Receiver Amplifier Board Red with Charging Port for Wireless Speaker DIY Sound System : Electronics

Battery- Amazon.com: YDL 3.7V 500mAh 502248 Lipo battery Rechargeable Lithium Polymer ion Battery Pack with PH2.0mm JST Connector :r