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I used to know a couple of guys who made snake cages out of 1/8 inch thick masonite. This is a composition panel that comes in 4x8 foot panels and is much cheaper than plastic. The stuff is too flexable to stand alone, but tacking or gluing it to 1x2 inch wood strips makes it much stiffer.
An incubator would need shelves and insulation. Wal-Mart and other stores sell metal shelf units. And styrofoam could be used as insulation. Though the foam tends to stink from plasicizer for months so it must air out for a long time before the eggs go in. Another possible insulation is half a dozen layers of corrugated cardboard salvaged from the Wal-Mart trash pile. Glass should probably be two panes with an air gap between them.
I've never tried making an incubator this way, so this is just brainstorming. And you are on your own for thermostat, heating and lighting. Good luck.