Its actually pretty easy, people in japan are doing it at home now adding cool pictures into the bottom area.
I cant read the Japanese text very well but what it shows they are doing is:
1. sanding the inside of the gaps.
2. cutting out pictures to fit the gaps ( drawings, magazine ads, photos, )
3. using putty and making a border around each of the two gaps.
4. filling the two gaps with a clear liquid ????
5. removing the border and sanding the excess dry liquid flat.
They say this method adds even weight of 10 - 18 grams to the putter head and it looks insane cool.
If you do a bad job this is what it should look like:
If you do it perfect it should look like: