Mark the new holes on the actual bracket with felt tip and take pic
with it on the frame.
forget about the middle hole as there is nothing in the middle to clamp
around and it will only bend the U bracket and plate.
Put a hole on each side of the 2 seat stays in the U bracket so you are using 4 bolts to hold it on.
maybe you can use the 2 outside holes.
That is a large diameter strong aluminum tube to clamp to so no worries
about crushing it.