1total_length = 240;
 2total_width = 180;
 3half_depth = 20;
 4border_radius = 20;
 5wall_thickness = 2;
 6chamfer_size = 5;
 7
 8module wedge(w, l, d) {
 9    linear_extrude(d) polygon([[0, 0], [0, w], [l, 0]]);
10}
11
12module half_box(l, w, d, r) {
13    translate([r, r, 0]) minkowski() {
14        cube([
15            l - 2*r,
16            w - 2*r,
17            d,
18        ]);
19        cylinder(h=undef, r=r);
20    }
21}
22
23difference() {
24    half_box(total_length, total_width, half_depth, border_radius);
25    translate([wall_thickness, wall_thickness, wall_thickness]) {
26        half_box(
27            l=total_length - 2*wall_thickness,
28            w=total_width- 2*wall_thickness,
29            d=half_depth,
30            r=border_radius
31        );
32    }
33}
34
35rotate([90, 0, 180]) mirror() {
36    wedge(chamfer_size, chamfer_size, total_width);
37}