So I started by experimenting plenty of solution for detection of the note. First thing first, I tested springs, it worked but had many constraints (especially weight handling and durability). 

I then looked at pressure detection, but again, I had problems to tweack it to work for a range between 15 and 120 Kg.

Then I came to the touch board (from a hackaday post BTW). this is a small board embedding an arduino, a mpr121, and a midi sound player. My first octave was born in my head.

I then searched what I can do for the conductive part for detection. i found plenty of very expensive solution, and finally came to this https://fr.aliexpress.com/item/Hot-Sale-100-Silver-Fiber-Fabric-Shielding-Fabric-Silver-Fiber-Conductive-Mesh-Fabric-Anti-static-Mesh/32762274779.html?isOrigTitle=true.

This is a sheet of fabric that is not blocking the light.

I tried to make this work, and created my first octave 

This was really a first run, all notes were separated, the design was almost there, but it helped me to understand what I need to do and more important what I need not to.