The PVC pipe idea is cool!
I got an old parallel port intelligent switch for $6 in a second hand store, and I've seen these before, for around the same price. Nice heavy metal box, with a polystyrene front plate (easy to cut or weld on) and some bonus parts like 25 pin IDC cables and a TO3 5v reg on the back. Also have a few data switches, which I got for $2 each - low price for a sturdy metal case.
Also snagged some metal cases - look like a drill index, with out the index - new for $3 each, also at a flee mart.
In all cases (ha ha ha) they are less than half what I pay for a cruddy-boxy aluminum case in an electronics shop.
I also collect small plastic boxes which look like they could be used. My logic probe is cased in the plastic tube from a complimentary (hotel or airline) tooth brush. (Box was better than the crap brush inside!)
There are some breath mint containers which look a lot like the small iPod cases, once you peel the label off.
Also have a collection of small tins, but I think they are too nice to cut up for project boxes, unless there was nothing else.