Thanks for posting this and making the model files available. I don't have a printer, but I can have a friend do it.If you have access to a 3D printer I designed several versions of the tonneau cover you can add.
Tonneau Cover for 1:20 Scale Rivian R1T by ironfan72 | Download free STL model | Printables.com
I also made several accessories to go with the truck,
1:20 Scale Rivian R1T Accessories by ironfan72 | Download free STL model | Printables.com