The basic material is the polyolefin packaging material I've used before on the field fortification and timber frame houses. Wonderful stuff, and since it gets thrown away otherwise, so very cheap!
A standard block of waste packing foam...
With all the bits removed that don't contribute to making it grubenhaus shaped! Blue foam (glue problems not withstanding), balsa or even offcuts of timber would all work as alternatives, albeit with more effort.
Purlins are added from cocktail sticks, attached with a good stiff layer of contact cement. You need purlins at both ends - I just hadn't got to the other end by the time I took the picture! The overhang of the purlins is given on the plan, but needn't be too precise.
The next thing to do is to add the roof, but that's a post on its own. Whilst you're clicking to the next post, this will give plenty of time for these bits to dry!