The Chandler Parsons contract is the standard for contracts that are hard to match. Three years with the third as a player option, and a maximum trade kicker.
The problem with offering that kind of contract is that if Harris declines his options after two years, the Celtics would only have his early Bird rights and the cap explosion means that if he is worthy of the max in the summer of 2017, the Celtics won't be able to give him what he wants unless it has cap room to do so. Of course, a lockout might change all that.