I did a similar mount, went to the hardware shop and grabbed a few thin sheets of metal (tin?) and cut some stuff out to make a shelf type thing. Mechano is pretty useful for this kinda stuff also, if you have some lying around from when you were a kid!
Your local hardware store should sell enough bits n pieces to fashion some kinda mount
Just have a horizontal bit and a couple angle braces coming off the 5.25" bay framing. Probably only need to drill four holes and attach 3 components.
Make sure you take out all the parts before working on the case, you'll end up with small fragments of aluminium everywhere/throughout components.