I order the Maxpedition Universal Belt Sheath and a 5" Tube sheath. They should be here be the end of the week. Both can be carried horizontal. So I'll drop and not back in this thread when they arrive and let you know what works and what doesn't. I'm really hoping that 5" tube sheath will work for the 16" asp. It says it has a max depth closed of 6.25". That's exactly the length of the 16" asp closed. So we'll see how exactly Maxpedition measures their pouch limits.
On a side not for the OP. I see a lot of people suggesting the airweight. In the 16" I would strongly urge you to get the full steel model. It's very short and you really need the extra weight to do any real striking. I mean let's be realistic. You don't want to have to use the thing but if you do you want it to hurt. I can see the airweights being of use in the 26" lengths because of speed. But that 16" is barely heavy enough in the full steel version. But it's pleanty fast as it is. I'd feel less secure with the airweight. Not sure if it would have any where near the impact the steel model would in such a small pacakge. God forbid I ever have to use mine, but if I do I want it to break an arm, collar bone, or knee in one whack to keep me safe. Get some training on how to use on of these also. And never hit someone in the head unless you're aim is to kill them. The right blow to the head with one of these can be fatal.