What I did to make one was just create a room, then texture it completely white or completely black. And yes the corners and edges of the room with disappear when you do this, but if you put another light in there they might re-appear. Those black and white textures should come with Hammer, or any source game.
I couldn't decompile the map, it just gave me an error. But I think it's only 6 blocks that are all a func_physbox. Just make them unmovable with the physgun and make them unmovable all together. The Coloring tool can color props, so it should work. Just give the blocks a white texture.