Wait...
WHY would you want to carry a knife there??? It's almost inaccessible to the wearer and also ridiculous to safely re-sheath without bringing the sheath out, it is quite accessible to others who are aware of its presence (or become aware), it isn't really all that easy to conceal and isn't noticeable to the wearer when it becomes inadvertently un-concealed (which is quite likely to happen in most scenarios where one is moving around)...
Honestly, any knife is MUCH more easily concealed AND accessible to only the user on his/her front or side. The only reason to put a knife in the small of your back is because you expect to be frisked elsewhere and don't anticipate moving in such manner as might expose the presence of the hidden knife. Why not put the knife under your belt-buckle instead? It is just as concealed, just as likely to go un-noticed, and is far more accessible and able to keep concealed. Why put a knife where you can neither tell if others have noticed it nor get to it yourself without some difficulty?