Just like you did in the first step to remove the person shaped voxels that we start with, use the Move tool to draw a box around the just the top of the chest to select the lid. It will help if you adjust the camera to be looking at it from the front and center of the chest.
Tip: 8 to Move
You can activate the Move tool to select and move (or delete) voxels by pressing the 8 key.