I would radius the bottom of the mounting ring to the same radius as the arch in the body or close to it. I would use a piece of sand paper face up using an object with a similar radius. I don’t like using the top of the guitar because you have to remove most of the hardware and could cause damage to the surface finish. You can make many makeshift items and first you need to find the radius you are looking for.

I often cut a piece of paper to fit inside the pickup cavity and hold it vertical inside the cavity. You can carefully trace the radius of the top surface with a pencil and look around the house for something with a similar radius. Look at old pots and pans, jars, buckets and even your car tires can give you a working surface to place your sandpaper.

After you find a suitable radius than you can carefully sand the bottom surface of the mounting ring. Having the mounting ring slightly curved will relieve the strain on the top of the ring where it cracks and make it have a better fit when properly fastened. The biggest problem is being caused by the newer plastics they are using. I wish they would use the older Butyrate plastics that were used during the 50’s and 60’s.