You need to do whatever it takes to sustain a consistent style across the painting.
Bear in mind that British Museum research shows that the average viewer spends an average of seven seconds examining a picture. So, consider how much attention will be focussed on this one percent area of your painting. This factoid suggests that the loose impressionists and abstract artists had something right.
It is dark here in Australia at the moment. Try as I might, I cannot see your studio setup from here. Have your studio light on by itself and adjust the dark fabric until the shadow covers the working space inside the shadow box. It may be helpful to extend the fabric horizontally before descending vertically.