Hi, maybe I can help! I was in the same situation whilst building my armor, here's how I did it. I built a slider system which basically locked the armor into place. Usually padding (like Glen said) works just fine, but if you're a lot skinnier than the armor, even 10cm foam pads don't cut it anymore. Making sure to have a good final appearance was key for me, so I went with the aforementioned sliders. Additionally you'll see an extra strap in the section below on how I make sure my armor stays in place with this adjusted strapping.
You can find some general tips on how to size your armor if you're skinny in this thread.
Feel free to reach out if you've got any questions and good luck!