Technically, extending a raffle is changing the terms, so that's a bit sticky, but should it be decided to go that route an extension of "reasonable duration" should be in place. I would guess one or two weeks might be acceptable, beyond that might be pushing it, it depends...
I don't know the terms of this raffle and am not trying to say what should be done one way or the other, I donated because it is a good cause and also kind of fun. Just mentioning it because next time there's a raffle you'd probably want people to have a good memory of them going well in general, however that's interpreted by the members.