Neither ABS nor PETG are dimensionally stable, so would be a bad choice of material. You need something that stays reliably flat. I've had good luck with copper-sandwiched phenolic PC board - very flat, stable, affordable, and easily machined, CNC or otherwise. Rocket booster engines are being 3D printed near here. You might apply for a job with Elon Musk and ask if you can borrow the printer for personal use after hours, although you might have to wait awhile. It allegedly takes several months to print a single engine; but unlike ordinary rocket boosters, they're reusable. For flat sheet products like neg carriers, CNC makes way more sense.