code:

desired_height=20;
desired_diameter=10;
chamfer_size=3;

module chamfered_cylinder_a() {
    rotate_extrude()
        difference() {
            square([desired_diameter/2,desired_height]);  // the main body of our cylinder
            translate([desired_diameter/2,desired_height,0]) // move to the top outer edge
                rotate([0,0,45]) 
                    square(chamfer_size,center=true); // subtract off a corner to get a fillet profile
        }
}

module chamfered_cylinder_b() {
chamfer_ht=desired_height-(chamfer_size/sqrt(2));  // our cylinder is this tall until the chamfer starts
chamfer_scale=(desired_diameter-(2*chamfer_size/sqrt(2)))/desired_diameter;  // the chamfer ID is this ratio to full cylinder diameter
cylinder(h=chamfer_ht,d=desired_diameter); // a full cylinder up to the bottom of the fillet
translate([0,0,chamfer_ht]) // move up to the top of that cylinder
    linear_extrude(height=desired_height-chamfer_ht,scale=chamfer_scale)  // extrude another circle
        circle(d=desired_diameter);
}

translate([-desired_diameter/2,0,0])
    chamfered_cylinder_a();
translate([desired_diameter/2,0,0])
    chamfered_cylinder_b();