Hi alohabob,
Most of the waterproof covers you can get now have the waterproof/breathable membrane which work quite well. They aren’t quite as breathable as natural fibers but they are the best compromise between breathability, waterproof qualities, and having little effect on the mattress itself.
They come in complete encasements (usually used for bedbug protection) or 5 sided with a stretch pocket that wraps around the mattress corners. Some examples of the breathable/waterproof membrane types of protectors are in post #15 here.
A slatted foundation is usually a “safety margin” for ventilation but a solid foundation would work OK in most cases as well. Even if you used a 6 sided protector it would probably be fine but the 5 sided version would be my choice unless bedbugs were a particular concern.
Phoenix