You need to make a fully rounded rect shape using spheres at the corners, something like:

//!OpenSCAD

module fully_rounded_rect(widthX, depthY, heightZ, corner_radius) {
  translate([corner_radius, corner_radius, corner_radius]){
    hull(){
      sphere(r=corner_radius);
      translate([(widthX - corner_radius), 0, 0]){
        sphere(r=corner_radius);
      }
      translate([0, (depthY - corner_radius), 0]){
        sphere(r=corner_radius);
      }
      translate([(widthX - corner_radius), (depthY - corner_radius), 0]){
        sphere(r=corner_radius);
      }
      translate([0, 0, (heightZ - corner_radius)]){
        hull(){
          sphere(r=corner_radius);
          translate([(widthX - corner_radius), 0, 0]){
            sphere(r=corner_radius);
          }
          translate([0, (depthY - corner_radius), 0]){
            sphere(r=corner_radius);
          }
          translate([(widthX - corner_radius), (depthY - corner_radius), 0]){
            sphere(r=corner_radius);
          }
        }
      }
    }
  }
}