Troble with meshes, offsets and thin extrudes

[Lars_luenebach]

Allright, storytime: I (absolute noob, selftaught aka no professionals to ask these questions) have been trying to create a something like this, but with a blender-made mesh as a base:

Problem is, the shape I'm working with looks more like this:

I only have the .stl of that second file, so I've imported it as a mesh, made a crossection, traced that crossection with the "Fit Curves to Mesh" tool, and created both an open and a closed spline path from that mesh crosssection.
So far, so good.
Now I'd like to create an offset around 15mm from the edge (only works with the open splines) and then extrude a 1,2mm thick wall from that offset (doesn't work at all).

I have no idea where to go from here. The error messages feel unhelpful, I'm not finding anything remotely close to my problem in this forum, reddit or youtube, and I've spend the last three hours trying in vain to figure it out on my own.

Thanks in advance for any help!

[jhackney1972 · Accepted solution]

I am not clear on what you want but I will say, you need to use something besides the spline tool in the "Fit Curves to Mesh Section" tool to get some better lines.  I have attached my model with a 15mm outside offset and then Extruded to a thickness of 1.2mm.  Please note the smoother sides of the model.

If you can give better instructions, I will be happy to help further.  Model is attached.

[Lars_luenebach]

I wanted to keep the edges as organic looking as possible, but since found that fusion is not the right tool for that.