Stone fort walls upgrade?

I have this ingenious idea that would improve the fort system, not that there is anything wrong with it. This idea comes in the form of forts being able to be upgraded with stone walls, which replace the wooden ones, in the same layout, i.e a tower at each corner and a gateway at each gate (obviously). I also thought that once this upgrade has been purchased, the fort then becomes permanent and will not disappear after one turn if no-one is occupying it, which may lead to tactical placements. This will generally create unbalance, so I want the walls, gateways and towers to be irrerepairable, to make their use as fair as possible, also a high cost will be implemented. The problem I have is i have no idea about how to do any of this, except for the layout of the stone walls. Any help appreciated. :)

I'm not really sure if you can do that thanks to Engine restrictions, yet I may be NOT right.

-A stone fort is already somewhere on the forum (iirc its actually even in vanilla, just unused)
-You cant upgrade forts
-You cant make ungarisoned forts stay

So basicly you cant do what you want, except make the fort a stone one by default.