The best method I have found that works for me is to work inside out. You work on the details first, the screw holes and the hole in the middle and then you spread outwards, completing the shape perimeter and fill in the space that is left that has no details. Then if the shapes is curved, you can try a shrinkwrap to be molded into a base curved geometry, or a simple deform modifier or just massage the shape pulling and pushing verts with proportional editing on.
