http://www.gargoylesoftware.com/lsp/moose is a pretty easy build especially when you don’t have help.
I agree with all of Sabin’s points.Coach the observers that they are to make no sound or talk and as much as possible no movement.Alternatively coach them on being temporary facilitators so they can appear to have a purpose as long as they follow your lead. Likely they will learn more from hearing the stories vs. watching builds from the edge of the room.
I would also agree that scarcity will inhibit expression and provide a distorted view of what LSP is offering. This may confuse builders and then the obervers as they will attribute behavior to the wrong cause.
At 3hrs and the numbers of folks you are trying to move through, you might find it a challenge. Watch this from Marko Rillo for ideas on large scale facilitation. He use Explorer kits for the entire exercise. You end up mashing the kits for the shared build.