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.
*Additional note:
The tang parts of almost ibuki blade blanks are made of soft iron or soft stainless. So you can change shape the tang with file or grinder. Please try setting the handle for your best method.
Please see these blogs too.
How to remove wa handle from a Japanese chef knife.