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!