Flip the bracket around and only use the center hole.
Then drill 2 more holes in the bracket on the inside of the seat stays.
Or you could notch the bracket in the middle removing the center hole
then re drill 2 outside holes higher up on the outside of the frame.
Then re drill the center hole below the notch.
Actually would be better to forget the middle hole and drill 2 more holes
on the inside of the frame so there would be 4 bolts, 2 on each side.
Of course you have to match the holes in the backing plate.