Comment by meric

What's the problem just telling it straight -

  Hi Bob,
  It's Jim at CompanyX. I discussed the interview with the
  team. It's difficult for me to say this and I don't know 
  how to say it properly, but we think you're pretty good but 
  doesn't quite clear the bar for us.
  We'd like someone who has more experience in X, Y, Z, but    
  in our interview, we thought A, B, C. I know our interview
  probably is no where near perfect and may not have 
  presented the entirety of your skills, but unfortunately 
  this is our interview process and we will have to abide by 
  it. 
  You're certainly welcome to try apply for jobs at our  
  company in the future.. XYZABC... Thanks for taking the
  time to interview with us, and good luck!

'Good fit' tells nothing to the candidate what they can improve, and deters them from applying at your company in the future when they further develop their skills and attitude.