I emailed Ali about this too and here is her response.
Thank you for your inquiry Sunny!! We do not recess an area for the belly heat as taking that much material out risks weakening the bottom of the cage. We do route probe slots in the bottom to accommodate your thermostat probe when using belly heat. The belly heat simply adheres to the surface of the bottom of the cage. There is typically a probe slot on the right and left of the bottom. Some will come with a probe slot in the middle too. Thanks again! Please let us know if you have any questions. Have a great night!