Theres a unity tutorial on it Learn Unity -Navmesh Runtime with links for the asset scripts you need as its in developement. All you need are the contents of the NavMeshComponents folder (both Editor and Scripts) and also the Gizmos folder if you want it. Theres also a useful vid demonstrating it here Unite Europe 2017 New NavMesh demo .
Then within set up a simple flow macro ie this attached to your object with the new NavMeshSurface on it.
The NavMesh will then be built ingame when the object is instantiated (or if its in the scene when its loaded)
And you can rebuild or remove the navmesh live by calling the Events like this
The git repository for it can be found here NavMeshComponents
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