1. You're right, to a degree at least. I believe the NIF does need to have a lot of this stuff already built in with flags and string data and such, but testing some stuff with the collision box mesh proves that the engine needs to be told to take advantage of these NIF features before it will look for them. Chiefly, if you don't use one of the editor buttons on the top toolbar to drop in one of these resizable boxes (collision box, trigger box, acoustic space, and so on), it won't have the Primitive tab in the Reference window. Additionally, it doesn't have a model if you select its base object in the Object Window. Meaning the engine does use that NIF but not the way we conventionally would. Also, setting it as the model for my activator gives the exclamation mark error mesh. I think I'll set this aside for now.
2. Oh? I didn't think there were any editor marker polygons there, so I grabbed the inner corner vertices in the vertex data and put them all at 0, <unchanged>, 23, effectively making a face on either side.
3. This worked flawlessly, thank you. I really wanted the outer edges to remain opaque but have a transparent rectangle face for the back and front, but I assume I'd need a second mesh for that?
Actually, I copied in your original mesh and love the result:
My internet is doing horribly this week, but when it's not so poor, I'll look into making resizable primitives for the editor, if we can even do that ourselves. For now, this is great. Thanks again, Pix. And you're always helpful, so I don't know what that last line is about.