I'll offer my two cents just because this is an interesting (and incredibly arousing) problem.

Seems like the tutorial icey provided would work great. Slip knots will tighten when you pull the rope taught and stay that way. You'd just need to use scissors to escape when the time comes. A knife as they mention probably isn't a good idea, just to be safe. Also, you might want to have a friend come by to check on you if you haven't called after a certain time.