Same as Alice said, Joe, Colorado is full of red rock sandstone formations which offer natural amphitheatres. If you don't have anything like that, maybe get some people to sit low and others make a ring around them a bit higher, so that their bodies help to contain the sound? Are most of your participants older with possible minor hearing problems which would be exacerbated by no walls? Maybe find a park with shelters whcih would help?

kat