Could you buy a few plastic storage bins and attach them together? Or you could use an old book shelf on it's back with the shelves removed.
For substrate you can use coco coir or top soil.
I have a small fenced in yard so I will have to see how big I can go while still leaving room for the dogs to run. He will probably stay outside most days and come in at night or when it snows.
I would love to see pics of your enclosure! Lol at trying to keep him from falling down the ledge!