I know this is probably a selfish ask, but of all the swords to wear naked at the hip, katana really are not one of them.
Longswords? Historical precedent for them, even rapiers and scimitar have scabbardless historical precedent, sword rings, loops of leather, etc.
Anyway, I implore you!!
It would just be a belt with a scabbard model that clips into the area of the blade, I think. Please do tell me if this has been discussed already.