OK, one solution I came up with:
block next to corner (vertical side) - big hst of sky, big hst of next long side of pyramid
actual corner block.- big hst of sky and small hst of each pyramid fabic across corner
block next to corner (top,horizontal side) - big hst of sky, small hst of each pyramid fabric, continuing pattern, but not completing next big side of pyramid
So you end up with sky as the three large hst facing the inside of the corner and partial pyramids continuing around.
Clear as mud???