I see from time to time people using random.org web site, filming sweepstakes. The solution is rather poor – you could film as many sweepstakes as you want, until you get a valid number.
The alternative? Create a list of persons, make it public, provide it to the web site, and let the web site announce it after a few days.
So, the mechanism is like this:
- 100 people join your contest.
- December 10 the contest ends.
- December 10 – you put the list on a web site, public.
- December 12 – the winner is extracted from the list and you announce it.
Much more difficult to arrange something, in this case.