Borderlands/The Salt Flats

The Final Piece
Defeat four bandit patrols outside of the central digging machine. Three are located on the south, while one is patrolling the north.

When the patrols are destroyed, enter the area from the west. Collect the weapons and head up the elevator. Defeat Baron Flyng, a level 30 raider equipped with a shotgun.

Once killed, he will drop Boom Stick, and you can open the weapon crate at the top in an attempt to get the key.

After you do, the outer gates will close, and you will not be able to leave by normal means. It is still possible to exit by jumping over walls.

Claptrap Rescue
The Claptrap is located in the north-east corner of the outer Thor base. The repair kit is hidden under a matress.

In addition to a backpack SDU, the claptrap will reveal a weapon crate.

Weapon Crates

 * Near the save point below outpost Thor
 * South of the outer entrance of Thor.
 * From the claptrap.