There is a way to do it, but you won’t like it. I discovered that foundations have a sort of build radius, in which they lock the area around them from being built on by the opposite foundations, or setting foundations in odd placements. It seems to vary based upon the layout, for instance an L shape would lock everything between the two ends of the L to form a box in which if you started with wood, any other wood foundations need to face the same direction.
To do what you want however, you start with metal first, and lay them out. Once done, you find a spot where you can start the wood that’s high enough to let you encircle your metal base. The Wood can get to be pretty big, but if you find the right spot you can make it so that assaulting the wood base just to get to the metal interior can be very costly. I found one setup for a 4x4 metal base which had nearly 100 wood foundations around it, and could only be assaulted from one side. People could of course jump on top of the wood base from the other three sides to get over it, but getting from it’s roof down to the ground would have been a fatal fall.