I'm not seeing what the big deal is really. Most guns throughout history were first designed by some tinkerer at home. Years ago, when I was into paint ball pretty heavy, my friends and I started making our own paint balls. For fun, and because we were kids and a little stupid and sadistic, we decided to fill some random balls with a mixture containing finely ground cayenne pepper. Now, every law enforcement agency in the world uses these. We also made napalm just for the hell of it, and potato guns (one of which took out a window and embedded a projectile in a wall). I built a battle bot weighing close to two hundred pounds with a flame thrower that shot 40 feet.
So now, someone with a maker bot, or similar, made a plastic gun. Big deal. I'm not near as worried about that person as I am the enterprising person who figures out how to make an untraceable uzi in a machine shop and sell it to thugs. Most guns are not particularly complicated. That's why you have smiths all over the USA, Canada, Australia, the UK, etc. who have been making firearms in their backyard machine shops for a long time. Most of them are licensed and totally legit. But what of the ones who are totally off the grid?
Really, I just wish the media would focus their attention where it might actually matter. In all this gun mania and fear mongering the media has been involved in, why haven't they been focusing on the gang bangers in south Chicago or the cartels killing people in Laredo and El Paso? Nobody in their right mind will go out and enjoy the river or lakes along our southern border because they might catch a bullet that was fired from across the border. And none in the mainstream press even bother to mention this stuff.