An interesting problem
I have not seen much written on this subject.
It all depends upon how you want to base your bonus structure. Typically it could be done either on the team meeting some kind of structure or on it being based on Management By Objectives, so that if the employee meets a number of targets they get some level of bonus.
The way it used to work in a company that I used to work for was based on a team target for customer satisfaction score.
It was something along the line of
95%+ CSAT score - 100% Bonus - 12% of salary
90-94% CSAT score - 75% Bonus - 9% of salary
85% -90% CSAT score - 50% Bonus - 6% of salary
75% - 84% CSAT score - 25% Bonus - 3% of salary
0 - 74% CSAT score - 0% Bonus - 0% of salary
It worked best as a team target as it helped to encourage team mentality and also did not suffer from the problem of low sample sizes which can be a particular problem if you based it on individual numbers.