For now I'm working around this with a cylindrical collision, instead, but can anyone with more experience with NIFs tell me why this object doesn't block actors from entering it?

http://ge.tt/1GdZP8L/v/0?c

Shouldn't bit 1 be set in BSX flags? For Skyrim that's the bHavok flag.

Thank you, that did the trick! Unfortunately, I forgot the sphere will be solid, so trying to stand inside it merely flings me hundreds of feet into the air But I've made significant headway with the cylinder, and I think it will work for me. Thanks again!