cosmetic crack filling repairs?

Hello everyone, im doing shackuhachi repairs for a while now, while most flutes only need strong bindings for cracks, some (like my main personal flute) have a crack that is closed by the bindings in the bore, but on the surface of the bamboo its usually about 1 mm wide.

i tried all kind of fillers while looking for something that can close that big gap without doing damage to the flute. last one i used was carpenter glue mixed with fine bamboo dust. it held fine for a while but after few climate changes the crack opened a bit the the filler loosened and begun to come out...

so im looking for a material that will be stable yet flexible enough to be able to take the crack's closing and opening throughout the year.
Ideas?

Itamar