This is the case without the lid on. There's still some sawdust on it. I wonder if I removed that before I mounted the motherboard.

The materials used are 16mm MDF board and 1cm dampening mats from a car part store. I also cut up an old (actually, new, but cheap) ATX case to get the place for the motherboard and ports. These are screwed to the MDF board. Also see here.