I don't know of any commercial products to do what you suggest. But if it were me, I'd just cut a sheet of thin metal or plastic to fit, bend it to the same contour as the pilot, glue it in place and paint it to match.
Actually, there's a bit more to it than that for a proper appearance.
However, if he is wanting to do that to a PW Lionel F-3 or a modern clone of it, with that smooth, unrealistic pilot, the detailing or lack of it on the cover won't make much difference. It can only improve it.
P&D is good; also, eBay has lots of detail parts. Search for "diesel pilot coupler cover" and the like.
Scale City Designs has O details.