Yes I asked a few years ago. The old ones were made of stone but most make them out of wood.
The idea is that once you have walled a fox in, you get your trap which is in effect a dark box open at one end, which goes into the den. (The trap door is also at that end obviously. The other end has a small slit to let light in and the trigger to drop the door.
The idea is that the fox enters the dark box, in its need to escape, scrats at the slit and sets the trap off.
The trap needs walling in properly to prevent the fox digging out. I don't have the plans but decided at the time not to