I feel like we’ve said this to OP already too, but:
…it came off that he was so popular…
However it may have come off, not enough people voted for him to win him the primary. He wasn’t that popular. For reasons mentioned elsewhere.
It’s possible some people who favored Sanders over Hillary voted for Hillary in the primary anyway fearing that she was more likely to win the primary and not wanting to chance unintentionally boosting the chances of someone other than Hillary or Sanders getting the nomination. I don’t know of any polls or anything that might have indicated that was or wasn’t the case. But that still means people didn’t vote enough for Bernie.
Far be it from me to dissuade anyone from applying the solution of 3D printong to any problem, but why not just buy one of those universal suction-cup-type flag car flag pole mounts and sticking it to the hatch itself?
Or maybe get a trailer hitch installed and use one of those flag poles that connect to a trailer hitch.
Mind you, those things I’m talking about tend to be made of steel. Definitely wouldn’t want your flagpole coming off on the highway or atop a bridge and impaling someone in another vehicle.
And, I’m not sure what legal considerations there might be for this venture, so it might be best to do your research. I know in my area, if anything sticks out too far out the back of your vehicle, you’re legally required to add a red piece of cloth or something. There are probably maximum lengths you may be allowed for a flag on your car.
If you insist on a 3D printed solution:
You know. Just… be careful about the whole endeavor.