Hey Welcome! 3D prints scaled up could be an option, but if you just need a slightly larger waist size a DIY extension is very simple.
You just need some scrap styrene or similar plastic material (You can buy styrene sheets in different thicknesses at hobby stores, or many plastic signs at hardware stores are made of styrene). Cut a rectangle extension to be glued on the inside of face of the armor and then another on top of that to bulk it up and make it look like it has the proper thickness. Use a seam filler such as Bondo if you want to make it look invisible, but even if it's not perfect the hip pouches usually cover this area.
I had to extend mine about an inch and this technique worked great. I found this illustration by TD2253 floating around the forum. Hope this helps!
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