I have slotted holes in whistles to sharpen them, and once to flatten a note by slotting the hole downwards and using tape to tune, in fact one of my D Generation whistles has a long C sharp hole, which I tape down to give me C natural to play in G key. It is not uncommon to see highland bagpipe chanters with tape and/or undercut or slotted holes to tune them in band situations. Sometimes desperation calls for desperate measures, always have the means available to both slot or tape holes when needed. Better advice than this I cannot give you.