t = 50;
w = 20;
wi = 15;
h = 5;
d = 1;

aw = 4;
at = 5;
ad = 2;

cr = 2;

module round_corner(r, h) {
  translate([-r, -r, 0])
  difference() {
    cube([r, r, h]);
    cylinder(h=h, r=r, $fn=50);
  }
}

difference() {
  union() {
    cube([w,t,h]);
  };
  union() {
    // rail negative
    translate([d, d, d]) cube([w-2*d,t,h-2*d]);
    translate([(w-wi)/2, d, h-d]) cube([wi, t, d]);

    // retainer hole
    translate([(w-aw)/2, t-at-ad, 0]) cube([aw, at, 2*d]);

    // round vertical corners
    translate([0, 0, 0]) rotate(a=180) round_corner(r=cr, h=h);
    translate([w, 0, 0]) rotate(a=270) round_corner(r=cr, h=h);
    translate([w, t, 0]) rotate(a=0  ) round_corner(r=cr, h=h);
    translate([0, t, 0]) rotate(a=90 ) round_corner(r=cr, h=h);
  }
}