Some internet browsing brought some answers about what has to be done with the cowls. I'm not really sure hwo it will look in the end.
First I use a ruler to mark the upper cowl's position on the exterior faces of the lower cowl.



Then I sand the interior of the upper cowl where it meets the lower cowl. The quality of the fiberglass is not very good compared to the wheel fairings.



After a few tries, the hole behind the spinner is looking round. On the pictures some 1/16" are still to be sanded.



Around the spinner, I have some problems with the joints. This one is not straight.



And on this one I have a 1/4" gap. I'll leave it this way as the sides of the cowls are still overlapping. I'll wait till they are cut and see what can be done.



Then I cut the slot in the lower cowl. This slot will later prove to be too short as the dimensions in the plan are given for a finished cowl, but I still have to cut some length of.



Then I made the cut at the back of the upper cowl. I needed quite some time to get it right as the first cut was much too shy of the line, so a lot of sandinf was needed.
I also drilled the pilot holes for the camlocks. On the picture there is a small gap between the cowl and the skin, this is only 1/16" and is for paint.



And here is apicture with the spinner, the gap with the cowl is on the mark.