Have you tried checking "Normals_Forward" where the option is available, on your material shader nodes?  The attached shows a quick test, using the Poser two-sided square prop.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.