The window is between booking and departure
A tour has one hard deadline: departure. Anything unsigned at that moment is either a guest you turn away or a risk you absorb, and neither is a good outcome with nine other people waiting.
That makes tour waivers a scheduling problem rather than a form problem. The link should go out the moment the booking lands — from your own system, an OTA, or a channel manager — with reminders on a cadence tied to the departure time rather than to the booking date. A guest who books four weeks out and one who books that morning need very different reminder timing.