The simplest way is to bury a piece in loose sand and pour molten Aluminum in or I've used green-sand packed lightly around the part. Here's a couple shots of an experiment I did last week with different foams. The green piece was a failure and the rest were about equal.
There is a lot of smoke and fire but you just have to hang in there and keep pouring. I have the metal hotter than usual and try to pour fast and steady or the sand will try to collapse.