My rear rack doesn't have a solid top, but in the winter (our rainy season) I just bungee something on top of it to act as a fender substitute. It doesn't keep all the spray off the bike, but does keep my back and rear end from getting soaked. Still need a front fender to keep my feet from getting soaked. A splash guard that snaps under the down tube can help if you don't want to have a full front fender. At least they have a concave shape so the water hitting them tends to bounce down and inward instead of outward onto your feet as it does when hitting the down tube.