Tomorrow, after the event is over, I will publish the result and secret string, and you'll be able to sha256("result-secret string") and verify the hash for yourself:

4349be1ba39869d64d60a375a88cb702540456040fce5cc830ec1eae4c75c376

This proves that the result is already determined and can't be modified by me.

You do have to trust that I'm not participating in the market, though. You have my word! It wouldn't be fun to bet on my own coin flip.