I'm not sure if the entire gun can be made out of plastic without making the gun near useless or explode in your hands as you shoot it.

Usually these 'homemade' guns buy a lot of professionally made gun parts such as the rifle barrel and just make their own lower receiver, which isn't exposed to the explosive high pressure part of a gun.

DemoRanch, Taofledermaus or iv8888 fired shotgun shells bare, ICRW. They’re the lowest chamber-pressure lethal (22LR and .25 are difficult but not impossible to make lethal) rounds.

Was the shot made of plastic too?

Most guns are already 50%+ plastic today so not really seeing your point.

Are they of sufficient plastic or composite composition that they don't trigger a metal detector? I don't believe so.

Disclaimer: I’m familiar with this only because I have to check my firearms when I’m flying.

Hard to say as all detectors are different. I only know that while as a teenager I got kicked out of public school for skipping class too much and was sent to an "alternative placement center".

Everyone there had to wear a white shirt and pants. There was only one entrance and there was a metal detector right in front of it. But what all the "cool" kids were doing was seeing how big of a knife they could sneak through it. And there was a couple of 3 inchers that got through. It mattered were the knife was and how you held your hands as you went through, but can't remember the details (this was back in 2001).

Also Hotel Artemis

https://en.wikipedia.org/wiki/Hotel_Artemis

The only completely polymer gun with a chance of not killing or maiming the operator is a shotgun cartridge blunderbuss with the shell/s place far away from the hands. With enough rubber bands, it should be possible to not need a metal firing pin.