Put up drywall against the corner so it pushes the corner of the drywall closer to the corner edge of the countertop? An octagonal sort of shape. Then you can

Put up drywall against the corner so it pushes the corner of the drywall closer to the corner edge of the countertop? An octagonal sort of shape. Then you can cut it off like a foot up or something and it can be a place to put a flower vase. :)