In forge mode, fly around the map until you find the corridor with the overshield powerup (red). Go to the right side of the power-up (facing the power-up from the middle of the map) and stop just past the doorway. Turn around to face the power-up and stack up items (red and blue crates work best, usually about 3 or 4). Jump on top of the items and you should be able to grab the skull if you jump into the missing space on the ceiling.