Since you are not planning anything structural, you might be able to assemble something from smaller pieces. Although a thin piece will be challenging there as well. However, if you were to split the piece in two halves (starboard and port) and were to hide the seam at the crown behind something, it may well look nice.
Not having seen a picture of the damage that you are trying to hide, there's still the unanswered question of why wooden trim would be the solution of choice. Normally, you'd just repair cracked fiberglass. Remind us again, why you think that's not an option?
In a parallel thread, I posted an image from another user that I found today rummaging around older threads. I'm copying it here again, because of the strategically placed inclinometer (or heeling indicator). Something like that would be a clever way to hide a vertical seam at the crown. Alternatively, you could simply leave a gap and insert a small wooden piece with decorative intent. Perhaps in a slightly contrasting color or grain, or with a different shape or thickness so it becomes a cap piece that stands out.
All of these strategies would help solve your difficulty trying to source a single piece.
Here's the image.
