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.