Far be it from me to dissuade anyone from applying the solution of 3D printing 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:

• I imagine you'd have to design it yourself. Even if you got help from a professional CAD kind of person, I'd have to think they'd have to be able to measure and work with your car in person.
• Mount it at multiple places. Trailer hitch and have the hatch hold it in place and connect it to roof rack bars, for instance.
• Use strong materials (straps, carabiners, steel, wood) for most of it and try to avoid having the 3D-printed parts take the brunt of the weight and/or stress.
• Take into account things like gradual warping due to stress and material fatigue.
• I know I'm harping on mechanical strength, bur make it bulkier than you think it needs to be and use 100% infill.
• Test it a lot for potential failures as best yoi can before taking your car out with the flag mounted on it. Maybe try some drives with only the pole and no flag first, then with a smaller flag before moving on to the real deal. Start on back roads and move up to larger streets and then to highways. Check for any signs of stress or warping between every test.
• Be willing to give up before endangering anyone. Better to live your life with an off-the-shelf solution or no solution than to be responsible for injuring someone.
• Be willing to scale down a little. Settle for a smaller flag, maybe.
• Consider how much this will affect your own visibility as the driver.

You know. Just... be careful about the whole endeavor.