Here's how I mounted mine. I used a couple of sheet metal screws through the top. (under the plastic of course.)
I first tried some double-sided foam tape but it wouldn't hold long term.
These pictures were taken before I was completely finished.
I also had to get an ATX extension cable to be able route the wires behind the optical drives.
The power cord routes out through the original plug hole in the back.
I also did have to invest in a Molex extraction tool. I tried other methods to remove the pins but finally gave in.
I think I paid about $16 for mine, I got it online somewhere.