My wife recently had a very similar incident outlined on another post (Broken Side Mirror). I wasn't keen on paying the $150 for a new mirror, especially when the rest of it appeared and worked fine. So I took it apart and got a sense of the design and function. I tried my best to create a 1:1 version.
The biggest difference I made, was that I made the middle of the part mostly filled for durability. There is a channel down the middle, that allows for the wiring harness to pass through. It also acts as an installation aid.
I've made somewhat of a walkthrough over on imgur: 2017 Chevy Volt Mirror Fix
You can also get a copy of the part here on thingiverse: 2017 Chevy Volt Side View Mirror Spindle
I installed this a week ago. It has survived a couple of hot LA days as well as a run through the car wash. I'm glad I didn't have to throw more crap into a landfill and save some cash too. Hopefully it helps you out as well.