Hi I have a 7 year old and I live in Cali. I used to worry constantly (and sometimes still do) because once we moved Hardy outside full time we got an insulated dog house, heat lamp, etc. and he hates it!! He refuses to stay inside. I used to hatch a lock to keep him in and raccoons out and he busts through it. Everyone says they need heat. Well Hardy doesn't like it. He wants his natural habitat. He goes up the embankment and burrows and seriously just takes care of himself. They are smarter than people give them credit for. They know when they need to eat/drink, bask in the sun or burrow. Yes I climb up every so often and poke him with a stick (especially with El Niño it's been pretty cold) and yep he's alive and he's stubborn and he comes out when he feels like it!! He's healthy and happy and my baby! I can't give you advice on the heaters because as I said he refuses to use them but I can say don't worry so much. Your baby is smart and he will figure it out on his own. Good luck!
This is wrong on all counts and your assertions are based on incorrect assumptions about the instincts of a wild animal that evolved in a tropical environment. They base their decisions on where to park for the night on a number of factors including security and where they perceive themselves to be safest from predators. Because there is no place that is too cold in the area where they evolved, temperature is often not a factor in their decision. They do not have a mechanism that tells them, "Hey, I'll be too cold if I stay here…" because there is no "too cold" where they come from. Conversely, they definitely DO have a mechanism that tells them, "Hey! Its too hot and my brain is going to cook if I stay here…" because it DOES get too hot in Africa, and here too, and they could die in the sun on a summer day. They DO know how to get out of excessive heat, but they DON'T know how to avoid excessive cold, or how to foresee that things will be too cold.
I can't even count how many sulcatas have died because people jumped to the same incorrect conclusions that you have now reached.
Sulcatas can certainly survive some cold temperatures in some situations. Because they survive these temperatures, does not mean that its good for them. This is a tropical species and the low temps that you are subjecting your tortoise to are not good for it. I know many, many people who have lost their tortoises because they thought the same way you are thinking.
I urge you to re-consider and put your tortoise in the warm enclosure over night. It only takes a few days or weeks and they get used to going in and out on their own. Appropriately strong latches are cheap and easy to find. We got some serious cold and rain coming in a few days, and your tropical tortoise should not be outside in it. We will be glad to share all the tips to get your tortoise used to its night house.