I too would go the natural shelter route - find the best natural shelter available, paying particular attention to drainage - you want your floor to be as dry as possible! Supplement the natural situation with debris and branches, using the ponchos where necessary. Pebbles or coins (anything small) tied in the corners will serve to anchor the ponchos. I might want to reserve one poncho for a floor covering if I can't find anything resonably dry.

If you build it right, you might not need the ponchos at all. I always prefer natural shelters, the result of living in the Southwest with its abundant and dry rockshelters...