another-take…

cube_physical_names.geo
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			h=0.333;

			Point(1) = {0, 0, 0, h};
			Point(2) = {0, -1, 0, h};
			Point(3) = {1, -1, 0, h};
			Point(4) = {1, 0, 0, h};

			Point(5) = {0, 0, 1, h};
			Point(6) = {0, -1, 1, h};
			Point(7) = {1, -1, 1, h};
			Point(8) = {1, 0, 1, h};


			Line(1) = {1, 5};
			Line(2) = {5, 8};
			Line(3) = {8, 4};
			Line(4) = {4, 1};
			Line(5) = {6, 2};
			Line(6) = {2, 3};
			Line(7) = {3, 7};
			Line(8) = {7, 6};
			Line(9) = {5, 6};
			Line(10) = {1, 2};
			Line(11) = {4, 3};
			Line(12) = {8, 7};

			Line Loop(26) = {4, 1, 2, 3};
			Plane Surface(26) = {26};
			Line Loop(28) = {9, -8, -12, -2};
			Plane Surface(28) = {28};
			Line Loop(30) = {-7, -11, -3, 12};
			Plane Surface(30) = {30};
			Line Loop(32) = {-10, -4, 11, -6};
			Plane Surface(32) = {32};
			Line Loop(34) = {10, -5, -9, -1};
			Plane Surface(34) = {34};
			Line Loop(36) = {8, 5, 6, 7};
			Plane Surface(36) = {36};

			Surface Loop(50) = {26, 28, 30, 32, 34, 36};
			Volume(50) = {50};

			Transfinite Surface "*";
			Recombine Surface "*";
			Transfinite Volume "*";

			Physical Volume ("PhysVol") = {50};
			Physical Surface ("Bottom") = {32}; // Bottom face
			Physical Surface ("Top") = {28}; // Top face
			Physical Surface ("Sides") = {26, 30, 34, 36}; // Side faces