bare/gpt-5.6-sol / tier-2-l-bracket

```openscad
$fn = 64;

width      = 50;
flange_len = 40;
thickness  = 3;

hole_d     = 4.5;
csink_d    = 8;
csink_depth = 2;

hole_x = width / 2 - 10;
horizontal_y = flange_len / 2;
vertical_z   = flange_len / 2;

eps = 0.01;

module bracket() {
    union() {
        // Horizontal flange
        translate([-width / 2, 0, -thickness])
            cube([width, flange_len, thickness]);

        // Vertical flange
        translate([-width / 2, -thickness, 0])
            cube([width, thickness, flange_len]);

        // Outer corner connection
        translate([-width / 2, -thickness, -thickness])
            cube([width, thickness, thickness]);
    }
}

module horizontal_countersunk_hole(x, y) {
    // Through-hole
    translate([x, y, -thickness - eps])
        cylinder(
            h = thickness + 2 * eps,
            d = hole_d
        );

    // Countersink from the outer/bottom surface
    translate([x, y, -thickness - eps])
        cylinder(
            h = csink_depth + eps,
            d1 = csink_d,
            d2 = hole_d
        );
}

module vertical_countersunk_hole(x, z) {
    // Through-hole
    translate([x, -thickness - eps, z])
        rotate([-90, 0, 0])
            cylinder(
                h = thickness + 2 * eps,
                d = hole_d
            );

    // Countersink from the outer/back surface
    translate([x, -thickness - eps, z])
        rotate([-90, 0, 0])
            cylinder(
                h = csink_depth + eps,
                d1 = csink_d,
                d2 = hole_d
            );
}

difference() {
    bracket();

    for (x = [-hole_x, hole_x])
        horizontal_countersunk_hole(x, horizontal_y);

    for (x = [-hole_x, hole_x])
        vertical_countersunk_hole(x, vertical_z);
}
```