I distribute packs solely based on GNW participation.

The more rounds you participated in then the higher chance you will get a pack and only the ones in the highest participation bracket have a chance at getting a hero pack. Every 4 weeks, I change it up and give packs starting from the lowest participation tier so that even the ones that don't always have time for GNW can still get packs. Hero packs are still only given to the highest participation tier, one going to the highest power and the other going to the lowest power. Whoever gets a hero pack can't get it again until everyone in the highest participation has gotten it.

Though this only works because my group isn't that big and we don't have that many people participating.