Will only 6 feet of cord be worthwhile? I've been using 'decoy anchor line' from Cabela's lately. It seems strong enough for a quick shelter and doesn't take up as much room in the pack as does 550 cord.
If that too large, say for a PSK tin, try 'suspended ceiling leveling line,' from Lowes or Home Depot. It comes in a plastic wrapped hank of about a 1/2 inch in diameter and 4 inches long, with about 450 feet of line if I'm not mistaken, and is surprisingly strong. I thought it would be fine for lashing one of those cheap plastic 'painters drop-cloths' overhead as shelter.