According to Sir Isaac, force is rate of change of momentum. An abrupt stop generates very large forces, which is why cars are designed with crumple zones these days. We need considerably less force to stop the stone if we change the momentum more slowly. Stopping it in say 1/10th of a second instead of say 1/100th second will reduce the force by a factor of 10. We could do this by binding something spongy onto the part of the block that will impact with the edge of the hole. A bunch of plaited rope (like they have on the front of tug boats for just the same reason) might be an authentic choice.

The Trilithon is on the army training ground (though no one seems to know exactly where). I've had one team of TV production company researchers looking for it before. At one time "Hit Wildlife" were going to feature my theories on their recent programme about Stonehenge, Unfortunately they decided to stick to the "why"? of Stonehenge and not the "how"? So I don't think we'll get a peak at it just yet.