To start, here's the process I used... no moulder, CNC.. just simple tools with a little patience.
1. I started by tracing out the inside of the window's curve on 1/2" MDF for my template that I would be using for each of the trim pieces. Then cut the inside arch with a jig saw, VERY CAREFULLY.
2. Now for the outside cut. I needed the template roughly 4" wide. Matching up with the profile of trim I had originally shaped for the bottom of the windows I made a little pencil gauge to make a line parallel with the inside curve. (Did that make any sense?)
3. After tracing and cutting the outside arch I sanded out any bumps or imperfections. Lastly, I counter-sunk some screw holes throughout the template.
4. Next I had to prepare my stock. I couldn't believe it when I went to Home Depot to find they didn't sell 1x22 poplar!! So I had to make ends meet (no pun intended) and edge laminate some poplar. I went on to screw my template to my stock, (keeping in mind strategic placement for best use of my wood) I then cut out the curve with a jig saw about 1/8"-1/4" from the edge of the template.
5. Now that I had my rough cut curve with the template still secured to the stock. I used a flush trim router bit to clean up the edge, duplicating the exact arch as my template. Because of the curve running across grains, I really had to take some time, avoiding tear out. I do have a few spots to touch up from learning the hard way.. oh well. You live and you learn.