I finally fought through the build process to make a plugin. It's based on the drawing plugin template but a bit more complete so maybe it may be useful to you.
dropbox.com/scl/fi/c8nu95x3r8mdjw15z7dcw/convexPolygon.c3addon
It renders convex polygons, and lets you change the array json of the points at edittime and runtime. Also when it does it changes the size and origin so that the bounding box/quad is as small as possible around the points.
I'm not encountering the bounding box issue you had.
When you add it initially it's white so you'll want to change the color right after to add it. I wanted to set the color with code, like the spritefont plugin does, but that doesn't seem to be an option somehow. Also it looks like you have to manually add ACES to set the blend mode with events.
I almost got used to modifying multiple files to add something. There really is a lot of tedium that you have to wade through to even get to the point of doing something interesting. The loop of modifying files, packaging them into a c3addon, loading it into c3 and restarting c3 to test it really takes a toll. Getting past the vague errors to even get the plugin to install was the biggest breakthrough I'd say.
I found the SDK manual helpful in parts but overall hard to navigate, but that was mainly due to the sheer amount of different classes. When I got the the runtime portion the SDK docs stopped being helpful and had to resort to browsing code in the debug console. Turns out at runtime the renderer function names start with lowercase letters while edittime starts with Uppercase. Also it found it odd that the draw function had to be done differently at runtime than edittime.