Hi swrstock,
I would generally use a cover that is the same height as the latex layers although you can also use one a little bit thinner if you want to compress the latex and change the feel of the mattress as well (it would be a little firmer).
The components post here includes the better sources for covers I’m aware of and includes many different options and thicknesses so you can choose the one that you believe would work best for the design goals of your mattress.
Phoenix