Cellar-in-a-box

I love it!

I love your dungeon in a box stuff. This one is excellent. If I was making it, I'd also put in a loop of lightly-elasticized string - they seem to come with vairous kinds of food packaging sometimes - to wrap around the lid (to hold the sides of the lid "together" when it's opened out like in the image), and likely stick a little weight to the inside lip of the lid (which would be on the outside when it's opened up) - say a coin or a washer - so that with the sides held square, the lid would sit "down" on the little prop (the circle of toilet roll) rather than bobbing up and down loosely.