Found this little spoiler thing that goes at the base of the rear window. It’s supposed to keep water from dripping into the trunk when you open it. Seems like it might help. Not sure about the fake carbon fiber look though. If it was just black rubber I’d consider one:
Tesla Model 3 Trunk Spoiler Cover Trim Carbon Fiber Water Retaining Wing 85CM | eBay
Tesla Model 3 Trunk Spoiler Cover Trim Carbon Fiber Water Retaining Wing 85CM | eBay