Hi gardenguy,
You can read more about mattress protectors in post #89 here, and here. A polyurethane film for the upper layer would generally be preferable over plastic, as the polyurethane film will allow for some breathability and be more flexible that the plastic, but still be waterproof.
As for the dimensions, both your cover and your latex will have some “give”, so I wouldn’t have much of a concern. Also, having a bit of a snug fit will assist with keeping things in place (although latex doesn’t tend to travel too much due to its higher coefficient of friction).
Phoenix