3d engraveing question

A couple of options in order of what I think is preference:

1. In an external application that handles raster images extend the size of the image with black enough that it will be larger than your planned mask

2. Create a shape that matches the final rounded rectangle that you have. Then create a circle around that which matches the mask circle. Put those shapes on a fill layer. This shape should now look have the shape of all your current white areas but now filled. Group and then overlap that shape with your current shape. If the dark areas of your image are not true black the transition to the vector shapes could be jarring and noticeable.