3D Model not showing all sides??

[fred_dot_u]

That was an interesting project. A large portion of your model had flipped normals. These are faces for which the parameters indicate that the inside is on the outside. I was able to fix that with Meshmixer, but the self-intersecting faces were a larger problem. For that, I fired up Netfabb, which eventually worked. I've not used Netfabb prior to this project and it has a great repair facility.

I've attached the repaired STL file for your enjoyment.

I strongly suggest that you consider to use a different program, even if it should require creating new synapses in the process. Fusion 360 is free to hobbyists, very powerful. There are many tutorials and videos on the 'net. Another useful program, also free, is Onshape. If you want the simplest possible program, try Tinkercad. Although it is limited in capabilities, it won't easily create problem files.

I'm a big fan of OpenSCAD, another free program, but I doubt that I would have been able to create your part using that program.

Good luck