You need two bars of thorium and a powerfully hot heat source to forge the key stem. You're on your own for the thorium, but I can help with the heat. To our west in Un'Goro Crater is Fire Plume Ridge - a simmering volcano. The lava lake up top is hot enough to fuse thorium and bone together. Your gold has bought you a mold that simply requires you to dip it in the lake up top; the lava will take care of the rest. That's it on my end. To finish your key, you'll need to talk to Arbington again.