Usually in a rectangular pattern. Try a fillet. Fillets are frame-type materials that are very thin, and are used to surround the visual area of the piece right next to the mat. Matching the fillet to the larger frame can be quite stunning. I use this type of technique for mega-pieces.
Some vintage drawings look nice with a gold fillet, neutral top mat, and matching gold frame.
I also love for bold pieces a gold fillet, richly coloured suede matte, and gold frame. It's up to you. I hesitate to use the term damage to any piece that can be repaired. For cels, paint that is cracked can be restored. That means that a professional restorer either rewets the paint and fills in missing areas, or removes the craking paint and replaces it with new paint that keeps the same appearance.