One of the great joys of wool is that it don't have to be close fitting. It can be HUMUNGOUS and still work. The only time it may not work is if it's too tight, and squeezes the nappy underneath. As long as everything over the top is as breathable as the wool (try to use natural fibres) you can put anything on over, and it will work. If you use a polyester sleeping bag, leave it open a bit, so the air can circulate. And if you have a waterproof mattress cover, lay a big fluffy towel between mattress and sheet, to again hep air circulation.