Making an Arch can be a difficult piece to build if you are trying to design a piece to fit into an existing opening. You must make a template which is usually no big deal but the more difficult
part is laying out your curved channel so it fits exactly into the opening. Through the years I have tried many methods and techniques, each with their strengths and weaknesses. I have found what I think is an easy method to set your curved channel so that it stays in the shape you need. I found
something called Arch Corner Bead. It is a bendable exterior plastic piece that contractors use on the exterior of buildings. I was driving past a construction site where the contractor was installing plastic trim around an arched window. I had an aha moment! He had a spare piece he gave me. He said he bought it from a store on line http://www.ebuildingproducts.com Arch Corner Bead.