Targus currently has this "belt" case on sale for $4 including shipping. I just bought one, it fits the n810 in its slipcase with some room on the side (about right for a thick pen, laser pointer, thumbdrive, etc). The n810 without the slip case is a bit loose. Not a thing of great quality, but the price is right.
Oh, nice find. While I was at CES, I found out that Case Logic has something similar, but I didn't get the exact details because I only found out about it second hand, and only after the show floor closed yesterday just before I was heading out for the airport. So I'm still trying to track down info on it.
So I'm still trying to track down info on it.