It starts out as a square block of wood. We made a pattern to make the top and bottom curve to match the cowl. We cut it with a band saw and then used an orbital sander to sand it to match. It took some time but it fits perfectly.
You can't tell very well but if you look at the top you can see that it is curved. It also has a slant to it to match the roof. We made this slant by setting it up on a joiner the angle that we needed and slowly cut away until it looked right. And then put a half inch radius on the front and sanded it to make it look smooth. We also cut the corners to match the curve of the cowl.
So to connect the side rails that are in the second picture to the header you cut a groove on the bottom side of the header and then cut the top of the side rails to go under it. This picture above is the header, it's cut out. And the picture below is the side rail with it cut out.