It's 5V, not 5.5V. Do not forget that, or you'll have to wait again, but for a new Raspberry Pi!
I think it could be pretty easy to do, since you just need a micro USB power supply, like a smartphone charger. You shouldn't have too much trouble finding one.
The Raspberry Pi is precisely designed to use them so it's perfectly safe, but remember, It needs to output exactly 5.0V and at least 0.7A. More than 0.7A is okay since the R-Pi just takes what it needs, but do not mess with the voltage. It has to be 5V and nothing else. You do not want to kill your Raspberry Pi.
For example, I have a Samsung Marvel S5560 charger, with a regular power socket, outputting exactly 5V 0.7A. It does not heat and is pretty small, and I'm sure it could be integrated in a case.
So find one, open it, and you'll have the perfect transformer. You could then try to put it inside your case, properly fixed and isolated, of course. Try to find one with a short cable. If you can't or if it's still too long to fit in the case, you'll have to shorten it. It's easier to weld the two thin wires of the cable on the transformer than on the USB plug. If you haven't any experience in welding, practice first. Also, welding thin wires is way easier with someone to help you holding them.