Also make donation opt-out instead of opt-in because most people don't care either way and with opt-out those who actually care have to take measures and not the other way around. That's how it works here in my and in many other European countries.
Turns out that doesn't make much of a difference in the numbers, because even opt-out countries tend to ask permission from the next of kin and the refusal rates there are higher if the dead person hasn't actively opted in and told people they want it.