You can always put a Y on the power, leave the case off, and let the drive hang loose Or, maybe not... If your computer can't handle it, you're kinda outa luck- I'd recommend just buying a slightly larger drive than you originally planned, and copy the contents of your old drive to your new drive. You can easily do this by temporarily unplugging the CD-ROM, and using those connections for the 2nd drive until the install is complete.
If you've got the room, get creative with a mounting bracket.
I didn't have a proper, designed, space for my second hard disk, but noticed a space underneath the power supply, so I drilled some holes in the bottom of the case, and screwed it to the floor of the case. Quite a few cases I've seen use this method, though they usually have pressings which lift it off the floor of the case a little to allow air to circulate. Mine seems fine as it is *crosses fingers*
Keep On Rockin' 'n' Overclockin' ;-)