Tiny change to jwhostetler's design. I extended one side of the rail connector by 4 mm.

Now it can be used to lock the drive in place!

Simply slide the drive in WITHOUT the rail installed, then push the rail through the right side first where it will slide in to the hole meant for the locking tab. Then line it up and slide it into the left side of the rails.

Now the drive can't fall or be bumped out without removing the rail first.