Semi-serious answer: Shapley value (https://en.wikipedia.org/wiki/Shapley_value) would provide a "fair" division.

Unfortunately it wouldn't be easy to actually calculate. The inputs to the formula are "how much of a donation would we have received counterfactually if the team had only included this subset of contributors" for all possible subsets of contributors. Though if it's a small number of contributors then you only have a 2^(small number) of counterfactuals to pull out of nowhere.