I'm not convinced, but there's a possible setup: a fund. Entities donate to the fund, and then the money would be distributed among the candidates (either equally, among a smaller number of qualifying candidates, or for all candidates, proportional to e.g. a party's representatives in the legislative branch)
(This would do nothing about money buying influence — as long as the donors are known, their wishes would be carried out by at least the major parties)
(This would do nothing about money buying influence — as long as the donors are known, their wishes would be carried out by at least the major parties)