Ski shops sell a product called P-tex. It's used to repair nicks and ding in skis. It's a plastic stick, 1/4"+- in diameter. You heat the end of it with a lighter until it melts. You can drip it on to the cracked/broken area or heat it until soft and "mush" it in. It comes in various colors, including black. It's easy to final form with sandpaper, a sharp knife, chisel, file etc. I've used it for various repairs, including end caps.