The Aopen XCube comes with a removable aluminium hard drive/optical drive rack. Once this is removed from the case, the hard drive can be screwed into its free 3.5" slot. The DVD drive should be screwed into the upper 5.25" bay that corresponds to the drive opening in the case.

The rack is then replaced in the case and screwed in. Most SFF cases use some form of removable drive rack system like this, but your experience may vary.