When a ready-made wa handle can’t be fitted for a tang or it is needed to adjusting the size, please take a look below for your reference.
How to expand a hole of a handle
We recommend a small-sized diamond file that has better cutting force than sandpaper to expand a hole quickly and carefully.
*In our experiences, a square type file would be better than a circle one for cutting and forming.
*Another Recommend items: for hard wood file, Jig Saw Blades
How to fill the gaps of a handle
Please use an epoxy resin or a bond that has high durability, do not use a wood glue as it’ll be melted into water.
If there is still a gap, kindly insert a thin wooden board or spread resin mixed with wood flour.
To fill the gaps closely prevents water entering and provides high durability. 
*Sample: ThreeBond epoxy resin
Please note with care: There are a risk that natural wood might be broken by hitting handle to insert a tang. *recommend use a wooden hammer or resin hammer
*Hard woods splits easily, please be careful.
Please see these blogs.
How to remove wa handle from a Japanese chef knife.