Bart as you correctly said I can use this as a starting point and I will
continue to investigate beziers.   However I think (could be wrong) your
solution only works if the two lines either side of the corner are the same
length... for example this does not work... but I say again it does give me a
start point to think about.

//Setup for corner points
#declare C1 = <0,0>;
#declare C2 = <4,0>;
#declare C3 = <4,16>;
#declare C4 = <0,16>;


//Create array given desired corner radius of 1
#declare A=array[8]
{
   C1 + <1,0>
   C2 - <1,0>
   C2 + <0,1>
   C3 - <0,1>
   C3 - <1,0>
   C4 + <1,0>
   C4 - <0,1>
   C1 + <0,1>
}

// Here some real cubic curves are used for corners

prism {
        linear_sweep            // or conic_sweep for tapering to a point
        bezier_spline           // linear_spline | quadratic_spline |
cubic_spline |
        bezier_spline
        -0.5,                   // height 1
        0.5,                    // height 2
        8*4,                    // number of points

        BezierLine(     A[0], A[1] ),
        Corner(         A[0], A[1], A[2],A[3]),
        BezierLine(     A[2], A[3] ),
        Corner(         A[2], A[3], A[4],A[5]),
        BezierLine(     A[4], A[5] ),
        Corner(         A[4], A[5], A[6],A[7]),
        BezierLine(     A[6], A[7] ),
        Corner(         A[6], A[7], A[0], A[1])
        // [open]
        sturm
        texture{Texture}
        translate x*-9
}

sphere { A[0] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[1] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[2] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[3] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[4] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[5] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[6] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}
sphere { A[7] 0.1 pigment{Red} rotate 90*x translate <-9,0.5,0>}

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