Hey Sparky,
Thanks for the question, I am going to give you quick answer then also provide a few links to some other posts from Phoenix on this subject.
There is always a tradeoff between the different functions that a mattress protector is meant to provide. One of these is the degree of water resistance that is important for each person and their circumstances. The second is the effect on the feel and performance of the mattress (thicker less stretchy protectors will affect the mattress more than thinner more stretchy protectors). Finally, there is the effect on temperature regulation and airflow (more airflow and moisture wicking will have less effect on the temperature regulating properties of the mattress than less airflow and less moisture wicking).
For your specific needs, you want to make sure you find a “stretch knit” cover without a waterproof membrane. The stretch knit provides the give and stretch to maintain the feel of a quality latex topper. Pay close attention to the size of the topper you have and the size of the cover as sometimes a very tight fitting cover will firm up the topper a bit too much.
Here is a link to a latex cover post from Phoenix about this subject and also more associated links within this article.
Also here is a quote from a post Phoenix had on 11/12/2018 (Post link).
Thanks,
Sensei