lengthParent = 30;
widthParent = 15;
heightParent = 10;
d = 2;
e = 1;
lengthChild = 5;
widthChild = 2;
heightChild = 1;

difference() {
    cube([lengthParent, widthParent, heightParent]);
    translate([e,e,d]) cube([lengthParent-lengthChild, widthParent-widthChild, heightParent-heightChild]);
    //translate([]) cylinder([])
    translate([0.75*lengthParent, 0.5*widthParent, 0]) cylinder(h=heightParent, r=1);
    // alle weiteren Objekte die hier stehen werden von dem ersten abgezogen
}