Thanks for the comments and questions.
The substrate and cap are each about 1.5 inches. If I had to do something different, I would have sloped the substrate and cap from the front to the back, up to 4 inches total maybe.
The gourami has space to breathe. He was a bit confused at first about the partially enclosed top, but he figured out where it's open for the filter/lights. He doesn't get along with anything, and the space seems plenty large enough. He really seems to enjoy the crypts.