Wool is a fab cover! I struggled with flip covers in the day so no way they would work for us at night. There are usually wool soakers for sale on the classifieds, or you could make some yourself, although tbh preloved covers will have already been lanolised lots of times and probably have some felting so are more bombproof.
As for the nappies themselves we use wnnn but not got an extremely heavy wetter. Hopefully some one else will be along with more nappy advice