Straw Bale Sheep Shelter

I'm hoping to get a small flock (1 ram and 2 ewes) of Icelandic Sheep in 2006 (if I can get the 6-strand, high tensile, electrified fence installed by then) and I've been considering shelter for them during lambing and so they can get out of the rain/heat, when necessary. Someday I'll get around to building a small barn, but for now It's got to be something small, easy to build, and cheap (my wife and I are building our own home now and there's little money for anything but that). We've got a fairly steep hill (a D slope on the Soil Survey map) that faces South/Southeast and I was thinking about cutting an alcove into it and making a sod-covered, stucco-coated, straw bale shelter, measuring maybe 4 feet (deep) x 6 feet (wide) x 3 feet (high). The roof would be supported by boards and waterproofed with scrap rubber sheeting. Around the back and sides, I'd backfill with drainage stone. I was wondering what you'ns thought about this, particularly the bit about it being earth-sheltered, except for the front, which would be exposed? I'm not sure how well the stucco-covered straw would last underground.