We can do this on-chain since the fees are pretty low right now. I was thinking of a simple 2/3 contract. If there's a disagreement between the bettors, the judge will decide. nostr:npub1qny3tkh0acurzla8x3zy4nhrjz5zd8l9sy9jys09umwng00manysew95gx could be the judge; I like him, and I discovered Nostr through him. If you agree and he accepts, it could work.
Discussion
Fees are low at the moment yes, but that can change in a moments notice.
Let me look around to see if there's a safe way to bet with LN. It should be possible with hold invoices, its just they don't usually hold for that long.
Yes, fees might increase in the future (they're likely to go up), but that's the cost; it's part of the protocol.