Protect the burrow opening from cold.
Your desert tortoise should be snoozing in ideal brumation temperatures now, 35°-50° F. However, the weather forecast for Las Vegas shows temperatures as low as 23° December 5-12. So, especially if we have wind, protect the air in the burrow from getting too cold during this cold spell.
You can wad up some newspaper, tape it with masking tape to hold its shape, and stuff it into the burrow opening. You can see that the opening is not sealed but allows some air circulation. Tad could easily push out the newspaper if he wanted to come out. Remove the newspaper bundle when the weather warms up a bit.