So, I have always wanted to turn my melodica into an electric melodica, after investigating a little bit I can create it in the easiest and cheapest way using just
3 components and some tools that we all have at home.

Supplies:
Materials: You will need...

melodica
piezo microphone (Contact Mic)
mini speaker (AAA or triple-A battery)
plug adapter

Tools: You will need...

pliers
wire
screw
spacers
screwdriver

Step 1: Piezo Mic

The first thing is to place the piezoelectric microphone, as my idea was not to pierce or modify the plastic shell of my melodica, take advantage and I removed this screw that is near the handle
and with a longer screw and some rubber spacers (which could also be washers) I put the microphone in place.

Step 2: Plug Adapter

Now we put a 3.5 to 6.35 adapter plug in our piezoelectric microphone...

Very easy!

Step 3: Mini Speacker

The following is even easier, we just have to place the mini speacker in the adapter plug.

Step 4: Place the Mic

Now we place the microphone in the right place, this is very easy since the microphone is self-adhesive, in case your microphone is not, you can use velcro.

Step 5: Be Neat and Tidy

And finally we take care to leave the cables neat and tidy with a few turns of wire, you could also use some security seals.

Step 6: Conclusion

This has been super simple, as you can see, this does not affect anything in the position or in the community when it comes to holding and playing our melodica, I bet you will enjoy this project a lot since the result is optimal. Up to the next makers!