Thanks, I!

Welcome to the site and good on you to start making giveaways so soon! But as noted above, you can't request a reroll unless the winner fails to respond for a week. If you want to maximize the chances of the winner being online to receive the gift as soon as the giveaway is over, you could make the giveaway duration the minimum of 1 hour. But that's still no guarantee. If you're waiting on winners giving feedback to get more slots, there's little to do besides being patient.

There is one other strategy though. You could make the giveaways private and create a forum thread where you offer to give the invite link to anyone who replies with the promise that they will be online and will activate the gift as soon as the giveaway is over. Still not 100% certain, but much better odds.