boring a hole into a mountain - nif flags look (almost) good, but what about collision?

[timstertimster]

Spoiler

 

As visible in the comparison between vanilla (left, and my mountaincliff01.nif), the surface doesn't look the same, but my flags and nodes look ok - what could be the cause of this?

 

Also: how do you generate a bhkCollisionObject? I have made a hole in the mountain and I need to be able to walk though it in-game.