After asking about, there seems to be two sites that might be good candidates1:
- Engineering
- Tag: CAD
- Tag: Solidworks
- Graphic Design
- Tag: 3D
IMHO, SE.Engineering would seem to be the best location, although it probably depends upon the practical application described by the question. What I mean is, if the design is for a logo of a company, then SE.GraphicDesign is probably a better fit.
There is a SE.Computer Graphics but that would seem to be more for coding and low-level stuff. SE.Mathematics is definitely a "no".
Blender
Obviously, if the question is about Blender then SE.Blender is a perfect fit. Blender is a special case as it already has a dedicated SE site.
External sites
Another alternative is Reddit, for example
1 As of yet, I have not had any "buy-in" from the mods or members of those sites. TBH, I haven't even contacted them yet.