This video explains one way to accomplish close to C2 curvature (continuous curvature) on a slot where the lines are tangent to the arc. You cannot achieve C2 curvature on tangent faces or line turning into an arc. Tangent faces by definition are C1 curvature. The idea is to use the slot as a master sketch and make a spline on a different sketch but the same plane as the master sketch. The spline will be a little outside of the master sketch but have coincident relations to the slot sketch. I will use zebra strips and curvature to evaluate the difference in C1 and C2 curvature.