We had a customer come in today that had a Ruger 22/45 MK IV Hunter. He found a universal Thumb Rest from TandemKross, but needed to have a rail added to the barrel to allow it to match up. There wasn't anything on the market though that was a direct bolt on where his thumb wouldn't interface with the ejection port of the firearm.
We took a piece of MOE rail and machined in a matching barrel profile. We then let him test fit its location and marked the barrel. It was tapped and drilled to match. It of course came out looking factory. All setup was done on the mill, squared to the receiver.