I wouldn't advise using p-clamps on a carbon frame. That seatpost clamp with threads for rack mount looks like the way to go assuming it is available and fits your frame.
I have often advised using the brake bridge hole as a top mount, but these days bikes are increasingly lacking them. I have also used canti bosses, but again they are not present.
If you hadn't already purchased panniers I might have suggested that you consider whether you might be happy with a rackless system. They work out well for some riders.