For it to be a cloud you might want to consider having your friend carry it around for you, install software on it, keep it running, answer your calls when you have trouble access it. To be personal, your friend would do this all for your exclusive private access. He would keep spares around so in case one Pi had a capacitor explode or something he would quickly and seemlessly swap you over to another Pi device and you'd hopefully not see any downtime or interruption of service.
I'm presuming mostly you want to operate in areas that don't have access to the internet because otherwise you wouldn't need the portability of your cloud (unless the machines need to keep on the move to avoid some other threat, perhaps Syrian agents, or an overbearing mother, or a rival gang of 10 year olds).
I wonder whether you'll need wireless access to your personal cloud or whether old fashion ethernet cable plus a hub will do the trick, perhaps you can bring down the expense by having a USB hub and the devices speaking to each other over that.
Maybe I've got the picture all wrong and your cloud will be more akin to a botnet with Pi devices geographically separated but all connected to the internet. This way if one site is compromised by said threats (or perhaps just a hungry rat with a penchant for chewing on circuit boards) your systems just seamlessly route around it. All data on your device would be encrypted and you enemies would be further confounded in their analysis of your device by strategic lack of access to the BCM2835 datasheet combined perhaps with the possibility that they are rat.
Certainly the Pi is great at firing the imagination.