Cool. As many have said, shame about you not being able to post any pics.
Here's the latest mods to mine.
The bottom box will be the next add-on which will be a big sandbox / play area of Buster
This is going to be joined internally to the top bin with a ramp. I'm going to put a hole in the base of the upper bin and the lid of the lower. There will also be a tub with a hole in the bottom, glued around the hole in the floor of the top bin to prevent as much bedding as possable from escaping to the lower level.
Good luck with yours, hope it has given you as much satisfaction to build as it is fun for Goliath to play in.