Comparing to index funds is neat. What Flattr.com offered was to set a monthly budget and then not allocate according to an index but according to your preferences (value per flatter = monthly budget divided by number of flatters).

Flattr is no more. But I could see that work out for open source projects: Allocate a fixed monetary amount per unit of time you want to donate. Record "intent to donate" during that period. This could be done via a browser extension or a CLI. At the end of one period, distribute.