Glue and smooth into place using paintable caulk eg Dynaflex 230. The tapered foundation should be recessed behind the trim. Paint to match the wall.
Includes 2 extra variations: "Short" lacks the tapered foundation and is useful for adding the transition to existing corners that were already filled in eg with caulk. “Tall” has a much taller tapered foundation which may ease installation by providing more surface to glue.
Printing note: The thin edges near the top will slice as floating bits but this doesn't seem to affect the quality of the print. The thin jagged edges will be blended into the wall.
The author marked this model as their own original creation.