Mujitsu and Tairaku's Shakuhachi BBQ

Happy New Year from NZ! I was slowly progressing with my playing but discovered towards the end of last year I was doing so with my tongue over my bottom teeth and touching the inside of my lower lip (apparently a common problem reading posts on this site).  It meant I was quite restricted with tone and also couldn't substitute 'tonguing' as a replacement for tapping the flute holes to repeat notes. 
I struggled to get a note with the new blowing technique - i.e. tongue behind teeth, I gradually got Otsu but despaired of every getting consistent Kan notes.  Then my teacher (Chikuzen via Skype) suggested narrowing the utaguchi end of my flute to improve the flute position - as I have quite a small space between mouth and chin.  I have a David Brown hardwood Shakuhachi and there was a ridge right around the utaguchi end.  I got a local woodwind repairer to slightly flatten the side opposite the utaguchi - and it has helped tremendously.  I don't know if this could help others who are struggling with this - or how easy it would be to slightly 'narrow' a bamboo flute - but thought it might be of interest. And apologies for all the graphic descriptions!