There are several methods to make sword scabbards. 

The two part carved method is one , it is possible to get this to fit very tightly just a lot of testing and fitting, most commonly if these are lined it's with felt. 

A second method of forming the core from thin sheets of wood (thin birch ply works wonderful for this) these sheets are bent over the sword in 2-3 layers per side and are glued , the glue bond between the layers will keep the form. This core can be lined with felt, and a welt added to make up the thickness difference. Most medieval scabbards were done this way, with thin verniers of wood with the grain running in opposing directions. generally lined in felt, then wrapped in Linen then in leather or the linen was sealed and painted over. 

A third method I have used was to cut a welt out of leather or thin wood  and glue thin sheets of to the welt then shape the outside. and wrap in leather. these work best for flatter cross section swords like viking swords with wide fullers. I normally line these with felt. 

MP

Thanks Matthew Parkinson I did not know the laminate method was used in medieval times.  I guess there really are no new ideas.