Locations on the picture are labelled with numbers related to locations listed here.
Location 1 (Red Dahl Chest)
This is the chest needed to make Smash Head head spawn. To get there, first navigate the Rat Maze (only accessible after almost finishing The Cold Shoulder) until it opens up into the Crystal Claw Pit. It is to the player's right as they enter.
Location 2 (Red Dahl Chest)
Leave the Crystal Claw Pit out of the second exit and head right. Make your way around the bend that follows until an elevator is reached. Activate the elevator to arrive at the Rakkcave. Killing Rakk-Man is optional however behind his spawn lies the chest.
Location 3 (Red Dahl Chest)
Exit the Rakkcave through the door to the right of the cave. Jump to the center platform, and then again to the other side, look to the right and check if the fusebox is active. If not jump down to the left and go to the left of the large door leading to the area with Sinkhole and go down the stairs. Once you reach the landing check the fusebox on the wall to the left of the door. If this one is not active go back up the stairs and follow the path downwards towards the exit to the The Highlands and once you reach the area where the rats spawn, look at the large door and turn left and this will be the active fusebox. Head to the building with the large door that heads to Sinkhole. once inside take a right and go up the stairs and go left, and there is the chest.
Location 4 (Red Dahl Chest)
Continue on the path after the area with Sinkhole, the chest is just before the end ledge to the right.