Why not just buy this?
https://www.amazon.com/Backplate-Radiator-Graphics-Backplane-Heatsink/dp/B09G9PLZD7/ref=sr\_1\_2?crid=2GU3KDVANWF8Q&keywords=3090+cooler+backplate&qid=1644275552&sprefix=3090+cooler+backplat%2Caps%2C92&sr=8-2