From what you have described, I imagine the geometry you want is 4 walls that are currently made of triangles, correct? And you want these to walls to be 0.1 m thick? What is your end goal here? Do you want to model them as shell elements that have a 0.1 m thickness, or do you want the full 3d model that you can support multiple elements through the thickness?

Somehow you have created a 3d body already (maybe with the Shrinkwrap). From here you have two choices. One is, backtrack until you just have the just outer surfaces. Then you will use the pull command (located under Design in the Ribbon) to obtain the desired thickness. The second choice is to use the shell (again under Design) command. Here you will select the two ends that will be open and specify the thickness (0.1). The software will then remove the inner ~80 cm.