It may be a good idea, but why anonymous? I’ve administrated several communities, and according to my experience, polls with a prize are very often abused.
For example, someone could ask all their friends to sign up just to vote, or even could register a lot of bot accounts to vote.
I’m not saying it’s going to happen, but it has a fairly high probability to consider it.
Sorry to meddle @SeanAlimov , but that’s a great point. I’d go further and suggest that only members who joined before the contest was announced can vote.
.