New double casement windows came from manufacturer bowed inwards at the center mullion so that window has an a very, very slight "V" shape when viewed from above.
The windows need jam extensions made so that they are flush with the adjoining drywall surface and must fit flush to existing jam, but the inward bow of the window near the center prevents the jam extension from making contact all the way across for the top and bottom extension pieces.
This means I can't make a jam extension with a flat surface fit the jam without a gap at one end (it rocks near the center). I've tried sanding down this area (see pic) but it is a tedious and I am not getting anywhere, and I am not sure this is the best way to do this. Feels like it would be better and easier to make the jam extension fit the jam but have no idea how to sand/plane/cut the extension surface to fit the jam. How would I do this? Is there a way to trace the jam surface onto a new piece so that I can cut it similar to scribing a bookcase to a wall?..I don't know how to do this if so!