OpenSCAD example of one way to chamfer when extruding a 2D profile

chamfer_extrude.scad

$fs=0.5; 
$fa=1;

// example usage
chamfer_extrude(height=10,ch1=-3,ch2=3) {
  hull() { 
    translate([-10,0]) circle(5, $fn=64);
    translate([10,0]) circle(5, $fn=64);
  }
  square([10,30],center=true);
}

// linear_extrude with optional chamfer on each end
// Parameters:
//   height - total extrusion length including chamfers
//   ch1 - bottom chamfer
//   ch2 - top chamfer
//    positive chamfer shrinks inward towards its end ("normal" chamfer)
//    negative expands outward towards its end 
// Limitations: 
//   - individual children of chamfer_extrude must be convex
//   - only straight extrudes with no twist or scaling supported
module chamfer_extrude(height=100, ch1=0, ch2=0) {
  eps = 1/1024;
  // in case chamfer*2 matches width of child object, don't make its layer zero width
  off1 = ch1 > 0 ? -ch1*(1-eps) : -ch1;
  off2 = ch2 > 0 ? -ch2*(1-eps) : -ch2;
  h1 = abs(ch1);
  h2 = abs(ch2);
  assert(h1+h2 <= height);
  midh = height-h1-h2;
  union() for(i=[0:1:$children-1]) {
    if (midh > 0) translate([0,0,h1]) linear_extrude(midh) children(i); 
    if (ch1 != 0) { // bottom
      intersection() {
        linear_extrude(h1) offset(delta=off1 > 0 ? off1 : 0) children(i);
        translate([0,0,off1 > 0 ? -eps : 0]) hull() { 
          linear_extrude(eps) offset(delta=off1) children(i);
          translate([0,0,h1]) linear_extrude(eps) children(i); 
        }
      }
    }
    if (ch2!=0) translate([0,0,height-h2]) { // top
      intersection() {
        linear_extrude(h2) offset(delta=off2 > 0 ? off2 : 0) children(i);
        translate([0,0,off2 > 0 ? 0 : -eps]) hull() { 
          linear_extrude(eps) children(i);
          translate([0,0,h2]) linear_extrude(eps) offset(delta=off2) children(i); 
        }
      }
    }
  }
}

chamfer_extrude.stl