It's not drawn as if it is entirely flat: it looks slightly tilted, like a blanket laid on top of a bed with a triangular prism underneath it on one side.
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It's hard to tell how a naked, terrestrial, subarctic endothterm would behave, since there don't seem to be any contemporary wild examples. Armadillos are somewhat similar due to their sparse hair, although they do not have fat reserves and these do.
The easiest solution is to say they migrate in latitudes or habitats prior to winter's approach in the Fermi Tundra habitat. Alternatively, you could specify they live in packed burrows as they hibernate, sharing body heat, and they stuff their burrows with insulating material (e.g., shrog hair). Low-density snow, such as fresh snow, is an effective insulator, while dense, wet snow is not so good an insulator. Therefore, you could specify igloo-like principles of construction whereby they use low-density snow. The
interior of an igloo can be 70 degrees (Fahrenheit) warmer than the outside, so, depending on the exact minimum temperatures of Fermi Tundra, that could be plenty to keep it above the freezing point (32 degrees Fahrenheit) and therefore not require antifreeze chemicals or similar high-cold-intensity adaptations.
More minor details include specifying it hibernates with its limbs, tail, and snout curled up, like how arctic foxes sleep with tails over their noses. A mother with young, depending on prone to freezing the young are (itself contingent on Fermi Tundra temperatures), might adjust the posture to share more warmth with her young. Perhaps a spot between her forelegs or hind legs could be like a brood patch in birds, and the young could press their vulnerable limbs or snouts against it for warmth.