For a portable case use a plastic footlocker, the kind with wheels and a hinged lid. This hinged lid was key for me as it allowed me to attach a keyboard, trackball mouse, and small monitor.
On the inside I have two ATX motherboards with dual core Athlon 64s, though I could have used anything had I felt like spending the money. The worker node has two graphics cards and an extra NIC for regular network traffic (the onboard gigabit NIC is used for message passing). The head node has an extra WiFi NIC as well for talking with the outside world. There are then two switches, one for each internal network, and two hard drives off the head node. The worker node boots off a USB stick. I found Ubuntu installed from a live CD provides a nice, small OS.
It's a little cramped (the top sides of the motherboards face each other), but there's enough room for the power supplies to divide the space down the middle, with the switches and hard drives mounted above that and opposite of one another. Everything is held in place with L brackets, plexiglass, screws and spacers. Between Newegg (computer hardware), Amazon (keyboard, mouse, and monitor), and the Home Depot (box and mounting hardware) the whole project only cost about $1,000.
What's really nice is that there's room enough in the box for four ATX systems with expansion cards, or probably eight-ten mini-ITX boards if you wanted to go that route.
If you haven't already, add these sites to your research:
They were extreemly valuable to me.
It won't be particularly easy, but it will be fun and rewarding like no other, and it makes a great mobile monster to show off to your friends!