Sky Box Box ( FREE )

Behold, the Sky Box Box, it's a skybox box, a full perm box with individual texture-able faces for the floor, walls and ceiling so you can make it look like any box you might like. Full perm too.

And you guessed it, it's 100% mesh (not that you couldn't use prims for this) you can choose if you want a black shell to give you privacy (people can still cam in on you unless you set your parcel privacy right) and it's got high LOD (it's still a box, even when you can't see it clearly).