My next instinct was to Google, "STL modifcation". After a brief scan of the results I had installed a few pieces of software and importing the STL--however the modifications I needed to make were nowhere near as simple as I anticipated. I scrapped the idea of modifying someone else's design and decided I would build this part from scratch.
I first experimented with Blender. I felt like a poet tearing pages out of a lined notebook every time I pressed the new project button. I'd get so far and realize that the way I was going about this (simple) design wasn't going to work or was more work than it was worth.
After realizing that my mind isn't wired to work with the hollow 3d design models used with Blender I turned to FreeCad. This took me back to my Middle School vector-based education in CAD--I can almost smell the musty Macintosh computers and remember the long rendering time it took to show the line art of the outline of a car wheel.
It took me a few more metaphoric sheets of paper to learn the FreeCad toolbox, but in short time I had my basic design complete.
The first print went very well, but I had reversed some key measurements and saw where a few design improvements could be made. After some upgrades, I had, for the first time, replicated a part from scratch. There was something rewarding in this accomplishment that could be told in a parable about teaching men to fish.
I also find it important to learn from mistakes---and sometimes mistakes come in handy, as you can see that the first few insert prints were almost a perfect reuse as insulators for my discone antenna build for my SDRs.
Now back to sawing wood.