The building of Fat Boy, the ice machine that Allie believes will ultimately save the natives from civilisation, is interesting for a number of different reasons. Let us remember that Allie has dragged his family with him into the jungle because he has had enough of civilisation and what it is doing to America. He therefore seeks to form an alternative home away from America in the jungle, taking with him the engineering knowledge of how to build an ice house.
It is rather odd to produce an ice house in the middle of a sweltering jungle, but if we think of the various religious imagery in the novel, we can see the ice as something of a scientific Lord's Supper or Eucharist, which represents Allie's honouring and worshipping of a God of science and technology who will save not only Allie's family but the Indians as well. This of course stands in direct contrast to the beliefs of the missionaries that are working with the Indians. His wilfull destruction of Fat Boy therefore represents his abandonment of this idea and his descent into insanity.