XPS 9570 cooler replacement

The surfaces are usually not perfectly flat hence one of the many reasons for thermal paste. But if you want to order a new heatsink assembly give Dell Parts a call.

Dell Spare Parts at 800-357-3355..have your service tag when/if you call.

Also..

https://www.waresphere.com/store/dell-xps-15-9570-m5530-cooling-heatsink-mv340-0mv340-at26w0010a0.html

Considered polishing with fine sandpaper and fine polish at the end? 

I'm not sure how much small irregularites matter, but it is surely important that the gap is thin and there is as much metal-to-metal contact as possible, because the best non-electrically-conductive paste is 10+ times less thermally conductive than the metals. 

Check alignment by firstly using tiny dots of paste near the corners, attaching and removing the heatsink, checking how the paste spread.