lc = 1;

Point(1) = {0, 0, 0, lc};
Point(2) = {2, 0, 0, lc};
Point(3) = {2, 3, 0, lc};
Point(4) = {0, 3, 0, lc};
Line(1) = {1, 2};
Line(2) = {2, 3};
Line(3) = {3, 4};
Line(4) = {4, 1};
Line Loop (5) = {1, 2, 3, 4};
Plane Surface (6) = {5};
Extrude {0, 0, 5} {
  Surface{6};
}
Physical Surface("back") = {6}; // z = 0
Physical Surface("sides") = {23, 19, 15, 27}; // parallel to z
Physical Surface("front") = {28}; // z = 5
Physical Volume("space") = {1}; // the whole thing
