I didn't look at the picture of his PC!
You can get bolt on bays and stuff, they're often pretty shit, but you could just leave it loose in the case. Just make sure that if it can move around, that it isn't going to cause any problems. also make sure the hardrive is at least partially secure as some are piss poor when it comes to getting knocks.
Like magicrounbabout said though, you could get an external case, they sell them on ebuyer for about £30. Only downside to this is that it will need to be ran through usb2 or firewire. pref usb2.if you don't have usb 2 or firewire it is not even worth considering unless you are just using it for backup and are not fussed by the unbelievably slow transfer speeds.
