@Ricky, I first tried my miking setup with the original plastic case and the key noise was very loud. Also, the capsules were too near the air vents and the amplification was not even. Now that there’s more space, I have far less key noise and the notes are well balanced coming out from the mics.
For the same reasons, I doubt an electric kit would be effective: there might be not enough space :/
@Melodica-Me, thanks! I used this http://www.saratoga.it/it/prodotto/1/212-impermeabilizzare/423-impermeabilizzante-z7/ and then covered it with a water based varnish, but it still smells of diesel oil… Luckily it’s slowly going away.
I’m planning to make a new case next summer, I’ll follow your advice.