[Therion] Pitch that is larger at top than bottom

Hi I have been trying to come up with a solution for the attached diagram. The pitch head is larger that the passage below, so if you turn off outline on the walls and put it on the pitch boundary, the fading goes but the parts where the pitch is on the outside of the passage take on the background colour so it does not look right. I have tied using invisible lines, or even tracing the walls to the survey above, but none of them look right, not sure if there is a solution, but suggestions welcome Andrew