Questions about homemade cave

I am thinking of getting a ball python and would like to build my own caves from clay. I drew a quick sketch of my plans and would like some feedback on it. First off how much volume should the cave have? I reckon I need to make several sizes for a growing ball python, but how many should I make, and how big should they be? Also I'm wondering a bit about the size of the entrance, I've read that balls get up to 4 inches in diameter is this so and how big should the entrance be?

http://i.imgur.com/yHar6l.jpg

The cave is in two pieces for easy access (one underground part and the lid resting on top) and the entrance is level with the ground. I am thinking of attaching a piece of driftwood to the lid as a handle and to hide the lid a bit (parts of the wood will be resting on the surrounding substrate as for extra stability if the snake decides to climb on it). It is going to be made out of clay and the substrate will be some kind of dirt/sand mix.

I would really appreciate if you guys could answer some of my questions or give me some feedback on my design :)

I think that is a cool and unique design, but it may end up being more of a hassle than it's worth. I tried making some (from dough though, not clay), and they were brittle right out of the oven, and very one broke after 4 coats of shellac with a little stress test. I make mine now from random sturdy plastic containers from the dollar store, and can make them in almost any size to fit my snake.
Also, I really like the underground concept, but it would be difficult to measure and regulate hot-spot temperatures if you're using a UTH with all the substrate and clay sitting over it.
Just my opinion, but it is a really cool design. Maybe others can help you figure out how to make it work if you're really attached to it though!

Yeah it would probably be easier to make it from some plastic container (you're not going to see the bottom part anyway) and cover the lid up so that you don't see the plastic. If i use a somewhat shallow bowl it will be easy to clean out as well. Also it will make having different sizes as it grows easier. So the underground part is all right then? (It should be no problem as they usually use underground burrows in the wild)

By the way, about how much weight should I put on top of the lid to keep the snake from lifting it up?

Thanks so much for answering my questions, I am crossing both my fingers and toes hoping that reptiles will be legal here in Norway soon.

It is a great design. I have a few comments, the shallow dish lower part would likely be better if it were a flat bottomed saucer shape similar to a deep under plant drip tray. They can fit is really small spaces I have female who is about 155cm long at about 4500 gm she fits in a 30cm by 10 cm round dish.

Make sure you do not have a fully closed circle as an entrance so if it get stuck (like after a big meal) it can still dislodge the hide and get in or out. Ceramic if left un glazed like a flower pot will hold moisture and if you were to soak it every week or so it would do wonders to keep humidity up.