I assume that for the 3d printed parts, you don’t need a high level of accuracy on the walls since you just want show a demonstrator object.
Therefore, one (very robust) approach can be to use the Dendro plugin to thicken the surfaces. The workflow is to sample a lot of points on the polysurfaces and then place voxels (e.g. spheres) on each point in order to create a thickened wall. You can export the placed voxels as a unified mesh.
Dendro plugin is here: Dendro | Food4Rhino
Download includes also example files for GH.
The idea of sampling a surface is described in the documentation e.g. see page 6:
Another approach could be to turn the polysurfaces into meshes (or one unified mesh) and then use the thickening component of Weaverbird or Pufferfish e.g.