I suggest you the method where you let your candidates pick a number from 1 to x. You have to run the giveaway atleast 48 hours. After that time when several members used a number, navigate to a website like random.org, generate any number, the number you see, which has been picked by someone is the winner. If the number is unpicked, please regenerate a number. Whoever uses a taken number, is disqualified from the giveaway. You can contact the winner via a conservation on the profile.
PS: This is the wrong forum, i suggest you to post it in "The lounge"