A model I think is interesting, though I don't favor it as a good fit for an international field / movement like all of EA: When talking to EA groups in Nordic countries (EA Norway, etc), I was surprised at the extent of membership and elections as the basic means of operating any group there. If I understand right, members pay a fee to be members, and the expectation is that everyone will travel to an in-person annual meeting to vote for various leadership roles. This helps address the question of who's involved enough to get a vote.
Coming from a geographically big country, it kind of blew my mind that everyone turns up in person at one location to vote. ("Surely you did it online during covid?" "Not in Sweden!") It's one way of making membership a bit costly, but it does favor members in the city where the voting is held.
I also notice that when I talk to people in some roles there, like the elected community contact person, they often don't have much experience because they were elected less than a year ago.
People with more understanding of this system would probably have more to say about its pros and cons.
I don't see a reasonable way to choose the voting population
I think that depends a lot on the specifics of the organization in question. For example: I think defining the electorate is a hard problem if the organization is devoted to spending lots of donor money. In that scenario, there are good reasons for people to seek a vote for reasons other than membership in the community.
But beyond that, most institutions in civil society do not impose demanding entry requirements. The US Chess Federation grants membership to anyone who pays a fee (and hasn't been banned for misconduct), without any concerns that the checkers crowd will stage a hostile takeover. To join a church with congregationalist governance (where the risk of hostile takeover is greater), you might need to attend a few classes, sign a statement agreeing with some core principles, and attend an interview with a group leader.
It's not clear to me why the techniques that work for the rest of civil society would fail for EA. Most candidates would pass on Forum karma, EAG/EAGx attendance, or other easily verifiable criteria.
I think it's quite simple for local and national community-building orgs, and maybe also international orgs as well.
Many civil society orgs use a membership model—opt-in, small fee, basic criteria—with a general assembly, which then elects the board. National EA orgs could do the same. National orgs could then send delegates to an international general assembly that would elect the CEA board.