The only truly "fair" thing I could do is forbid people from winning twice before everyone else gets a chance to win. But I have no way to prevent past winners from entering again at this time.
Although I can probably do this by adding winners into a new usergroup and not give them permission to enter until I do a reset.

Maybe I could just do a second raffle for this kind of purpose instead. But muh wallet.

Could also always introduce other forum games.