If you just want to play the ringing sound of the bell, a simple 3W speaker hooked up to one of your output channels will be good enough.
Here is a great step-by-step tutorial for the amplifier by Adafruit.
** Remember to use line from the MP3 Maker Shield to the amplifier **
So because this was a '90s boombox I was purposefully going for a LOFI '90s sound. I degraded all the sound files to 128kbps and ensured there was clipping in all of them. I really wanted to stay true to the era. Gluing the speaker and adding a scrappy cardboard/felt enclosure is not going to help with that HIFI sound you are probably after. If you have any tips to make the sound HIFI, please share them in the comments. About 60% of the decibel level was lost inside the enclosure because I drilled my speaker holes way too small :( That said, the volume level definitely goes high enough to annoy everyone in my office with dreadful '90s music.
** Drilling cast iron is slow. Recommend a good drill bit. **