I had been contemplating for a while how to fit the oil filler hatch since this detail on the cowl was molded very shallow and unsharp: just putting it on would have resulted in a bad fitting hatch protruding above the surface of the cowl. I thought of sharpening the recess with a handrouter or a file, but this was way to complicated. Another idea was cutting away the recess and inserting a new backplate in aluminium but then I would have to cut in the core material and I did not like that. So here's what I did: I made the hatch slightly smaller with rounded corners and deepened the recess with sandpaper until the recess was as deep as the thickness of the hatch (check with a ruler). Then I pinned down the hatch with 2 clecoes and, after rubbing in the edges of the hatch with a silicone release agent, applied filler. Sanded and repeated until the surface was perfectly flush.