My mantle was too thick to work with my store-bought stocking hooks, so I made something that looked nice and fit our mantle. The inside diameter of the main loop (thickness of the mantle) is 94mm. The inside diameter of the hook on the bottom is 17mm. The hook is 5mm thick and 10mm wide.

Approx. dimensions: 152 x 126 x 10 mm

I did a test print with PETG that was 2 perimeters with 20% infill, it held an empty stocking but flexed a fair bit. I would suggest either more perimeters or more infill- my second print I just set them to 100% infill and its great, they feel sturdy with little flex.