part_x = 60;
part_y = 40;
part_z = 12;
gap = 0.2;
base = 4;
wall = 6;
finger = 20;
difference() {
cube([part_x + 2 * gap + 2 * wall, part_y + 2 * gap + 2 * wall, base + part_z]);
translate([wall, wall, base])
cube([part_x + 2 * gap, part_y + 2 * gap, part_z + 1]);
translate([(part_x + 2 * wall + 2 * gap - finger) / 2, -1, base])
cube([finger, wall + 2, part_z + 1]);
}