Where are you located? It makes a big difference in the information we send you.
My sulcata lives outside 24/7/365. He has a heated house that he comes and goes from at will. His fence is about 2.5' high and there is a cement border under it. But he has never, ever tried to dig under the fence. Never! In fact, none of my turtles and tortoises have tried to escape by digging under a fence.
I would think that 6" would be deep enough to discourage digging.
I just rest slupmstone blocks on the surface. Mine seldom try to dig. If you give them a decent size pen in relation to their size and a good shelter, they usually don't try to dig. There are exceptions of course. What are you using to build your pen?
Thanks for the response we live in Escondido CA in the san diego area
i was thinking of using cement blocks with a wood fence on top of that because of the dogs we have.