"not" and the only reason that I feel that way---you will want to educate yourself on as many aspects of brumation as you possibly can before you support tortoise brumation...and winter is simply too close for you to become skilled enough to offer a successful brumation..
Closed chamber is the way to go. Important to note that desert tortoises, it is estimated by people who study them, spend 85-90% of their time in burrows, underground, where it is pleasantly warm and humid. Out and abouts is only for food and to warm up.
And ...
Welcome to TFO! Lots of information for you to learn from and always ask. Lots of great people and many experts who will do their best to help you with your concerns.
Desert tortoises are jewels. You are lucky to host these babies! : )