Ingress Protection

I don't have any recent experience with plastic casings and IP 64, but I am working on a metal case with IP66+ The material under consideration should not matter as long as it's not porous.

There are references for IP ratings online. Here is one:

http://www.machinedesign.com/ASP/strArticleID/56914/strSite/MDSite/viewSelectedArticle.asp

One important thing to consider is subtle: when you make something water-tight (IP67+) that introduces a problem -- moisture already inside the container (humidity) will be trapped inside and will cause problems with condensation. Also, plastics can actually pass moisture though to the inside over long periods of time and get trapped. In these cases a vent hole or semi-permeable membrane should be designed in, to allow for propert release of moisture.