Just like you did with the bottom of the chest, we need to remix to create another new voxel object, that will be the top of the chest.
- Open your original chest from your profile.
- Click the Save as a new object button.
- Rename it to Chest Top and click Save.
- Open and refresh your profile to make sure that there are 3 voxel objects for this chest, and that you aren’t just overwriting the original.