I'd spell it with an "a" too, but that's because it's my nephew's name also and that's how his name is spelt! I know how you feel though, Kieran gets his name spelt as Kieren or Keiran or Keiren...gah!
Hope you can make the meet up tomorrow, I'd love to meet Zach
Sorry, my initial instinct was Zachary, but then I would do Zach for short. But I'm always one to ask spelling before I write it, as there is so many ways of spelling names now (learnt this particularly from working in a bank). I had a surname which was always miss spelt (and it was a spell it as you would say it name)...at first it was frustrating but it became no big deal. I would say just point it out everytime they spell it wrong, eventually those people will learnt to ask everyone the spelling of there name (like I learnt to do).
It's annoying, and it happens with so many names. My DD Coco gets Cocoa all the time. NO she is not chocolate powder...............GGGRRR. I just end up spelling all our names every time!!
Bookmarks