The one that looks like a rocket head or a projectile tip. I hoped there was a corresponding LEGO piece. There is no inventory on the instructions.
Even though there's no corresponding LEGO piece, you could get away with this substitution:
This is how it would look with the pieces you have, and color wise the Horn comes in metallic silver too.
Bar with Light Cover (Bulb) / Bionicle Barraki Eye is close to the shape you are looking for. Can be placed over the round 1 x 1 brick. It comes in Light Bluish Gray color too.