As long as you leave enough space between the bottom of the existing HD cage and the added hard drive for ventillation, it's no problem. You basically make brackets to strap the second drive under the first with about 5 to 10 mm space between the added drive and the cage frame or drive bottom. You need only pay attention to possible interference with existing installed equipment (cards, speaker, front intake fan, etc.) The metal is conductive and capable of grounding the drive frame well.
Rinse the metal off to avoid any sticky soda deposits in the system. Aluminium is quite corrosion resistant, and convenient for this task. If you wish something stronger than comon soda cans, use aluminium roofing flashing material or (if you can get it) 0.3mm newsplate, the material used for offset printing of newspapers. You can also use light sheet steel, but shaping it is less convenient.