They were made of wood
Sorry had to do it. They are made from planks of wood that are 6x1 inches cut to two foot long then all screwed at each end to make a square. Then a piece of 9mm ply wood cut to the size of the square and screwed to the bottom and another piece the same size attached to the top with some hinges and hey presto a box. I also in some of them put a pieof the 6x1 down the middle to divide in two so if I'm putting torts from different groups they can be kept separate.
For bedding I just use chopped straw.