I wonder if a large fender washer would fit over the timing belt cover. It would be bonded to the cover somehow. The crank shaft would go through it with a tight fit, preventing rubber fragments from a chewed up belt from getting inside the timing belt compartment.
It is frustrating to try and do everything correctly only to discover that more was needed but you just didn’t know. There are lots of things like this in our modern world. Like the guy who followed the advertised 10,000 mile oil change interval with his Toyota and had the dealer do it, but they probably used cheap 5000 mile oil. His engine needed a rebuild at 200k miles. Some of these things are safety things, like the rear axle on the Ford Windstars from around 2003 that trap salt water and crack suddenly.