Write the call. Commit its fingerprint now — the text never leaves this page. Reveal it after the outcome, and the receipt proves when you called it: sealed envelope first, opened after the game.
What a receipt proves: the exact text existed no later than its witnessed checkpoint, in one history everyone sees identically. What it doesn't: that the call was right, or that its author didn't also commit the opposite call — one receipt proves timing, not a complete track record. 10 commits/hour per address.