another option (made the cube little thicker to make it more clear ) there are many more options in the RegionPlot3D you can adjust also.
cub1 = Cuboid[{0, 0, 0}, {20, 6, 20}];
cub2 = Cuboid[{12, 0, 8}, {17, 6, 17}];
r2 = RegionDifference[cub1, cub2];
RegionPlot3D[r2, PlotPoints -> 60, Mesh -> 10]
