We used a tent groundsheet that Tom brought along, going to buy a few of them. I thought it might not be thick enough to block the light fully but it really did the trick nicely.

Here's a shot of a dagger of light hitting the basin in Cairn L from same morning (the direct sunbeam landed nowhere else inside the passage tomb)

http://i306.photobucket.com/albums/nn245/wacamexhibit/2517CairnL.jpg