I was thinking about making a similar post on here and there's no reason it has to be restricted to sports.
This could work very simply:
  1. Let people create the bet e.g. Lakers vs Celtics.
  2. Then it's open for anyone to place money on one side or the other.
  3. After the event, payout the full pot to the winners in proportion to their share of the total wager on that side.
Rather than arbitrarily setting odds, just report what the payouts would be in real time based on the current amounts wagered.
As long as people are allowed to withdraw their wagers, there isn't any risk of being stuck with odds you don't like because of changes after your bet.