HD accessory catalog/website lists many chrome handlebar gauge mounts. From 2" (most common for oil temp) to 2 5/8". Some are real nice, and not stupid expensive. As you don't need to match your gauge face with existing ones, I'd suggest anaftermarket gauge over HD. They are lots cheaper, and more important, they have better increments. Get one with 300+ degrees min.
Google search "oil temp. gauges".