In 2007 we were without power for 11 days in January due to an ice storm. I have every kind of quilt and blanket known to man here. On the second day, I wrenched my back so bad on the ice that I could barely walk from the living room (with a fireplace) to the bathroom, so spent 10 days on the couch. The very warmest quilt was a double thickness fleece blanket that a friend had given to me. Mine was tied on all four sides. And when it was washed and dried in a dryer, it came out kind of wonky.
Making yours a king size would require seams as I don't think the fleece comes that wide. Don't know what you would use for batting though.
HTH