(For TF2) Hats, hats, hats! If I was entered automatically into raffles after watching an advert, and the more times I watched an advert, the more times I was entered into the raffle, I'd watch them all the time. Win, win for all.
Alternatively, you could record who reaches the threshold percentage, and give all those players a few non-game-breaking perks on the server, like aesthetics etc. That'd cost you nothing, and the user would get some nice features as a way of saying thank you.