Whatever you can find strap everything to. My personal favorite is the guy who put his parts in a milk crate he found/took. Looked pretty good actually. Said he even went back to the store to get two more for his seat to sit on.
My all time case mod was the guy who made his out of chicken wire and PVC pipe. He then strapped a giant fan to the side for the ultimate in air cooling CFM. You'd have to buy all that so its not $0.
Custom made glass case. Just like aquariums, only a little bit more complex. The case itself is technically free, but the components and materials to build it aren't.
As a matter of fact, that's just the thing I've been thinking about. Gonna look cool if I can turn it into reality...