There is a white oak in a park in the Bronx. Were it in England, people would be making pilgrimages to it. As it is, kids were climbing inside and setting it on fire. They had tried to block the entry with wood, but the kids had just broken in again. They asked us to come up with a solution.
We built a wall, but we built it inside the stem, so that the tree could continue to grow and to occlude the large hollow normally. On the surface, we placed a skim coat of concrete, and a very fine decorative artist color matched the oak bark and make shallow incisions to mimic the pattern of fissures in the bark.
It has protected the tree for three years now.