https://www.reddit.com/r/3Drequests/comments/1lugvrh/help_me_cut_and_key_this_stl/

// FishLid.scad

// A binary operator to remove something,
// is with the "difference()" function.
difference()
{
  // Move it, to put it centered on the x-axis
  // The command for moving is translate().
  // The numbers are found by visually putting
  // it in the right place.
  translate([-33.655,320.675,0])
    import("FishLidFull.stl",convexity=2);

  // The bottom of the rectangles is
  // 3.8 mm above the xy-plane.
  // That is the same as the edge
  // of the cutout in the lower-middle.
  translate([0,0,3.8])
  {
    // The height of the removed rectangles
    // is a "don't care". It should be high enough.
    // A height of 5 is high enough.
    linear_extrude(5)
    {
      // Make two of them.
      // One normal, and one mirrored.
      for(xmirror=[0,1])
      {
        mirror([xmirror,0,0])
        {
          // small vertical on the side
          translate([361,184])  // x, y
            square([13,44],center=true);

          // Big one in the middle upper.
          translate([0,246])
            square([100,25],center=true);

          // Small one in the middle lower.
          translate([0,137])
            square([60,12],center=true);

          // Top one of the four.
          translate([223,290])
            square([70,26],center=true);

          // Second one of the four.
          translate([223,242])
            square([70,26],center=true);

          // Third one of the four.
          translate([223,115])
            square([70,26],center=true);

          // Fourth and bottom one of the four.
          translate([223,38])
            square([70,26],center=true);
        }
      }
    }
  }
}