When the shape that you want to fill is vary small, then it is difficult to achieve a clean look with a regular Padded Satin Stitch. Instead of outlining the shape with Outline Stitches, use the Split Backstitch, with very tiny stitches. This stitch pins the outline to the surface of the fabric firmly, which helps the edge of the shape to be clean. Because the shape is small, it's enough to pad with 3-4 Straight Stitches, worked perpendicular to the Satin Stitches.

Note that in this drawing, the padding is shown in a different color for clarity. In reality, pad the shape with the same thread that you use for the Satin Stitches.