I had some scrap 3 mm plywood taken out of grocery delivery boxes and I wanted to put it to use. I printed some pieces of https://www.youmagine.com/designs/xyz-connect, scaled down to fit 3 mm panels, but found it difficult to make bottom panels that stayed flush with the sides of the box. I also had no use for the screw holes. These corners mimic XYZConnect but are thinner and designed with small boxes in mind.