You say that you would rather have your sewing room set up in a room with lots of natural light, and I agree, nothing is better than natural light. Instead of putting your sewing room in the basement, why not put your sewing room upstairs in a unused bedroom and just shut the door to keep the cats out. They may not like being shut out, but they will get used to it. Besides, won't the cats just walk down the stairs to the basement anyway?