I have done all 3 of these. I started by doing nothing, just refilling the airlock as stuff got sucked in, but after a couple of lapses I decided that was too much trouble. Once I just sealed it shut, but I got a bit alarmed when I saw how much the sides of the fermenter buckled in. It is flexible material, but less flexible when cold and I worried that in the long term this could cause cracking.
Now I have a top with two holes: one for the airlock and one for a mylar balloon. The balloon hole is plugged with a small stopper until fermentation is chugging away and I feel that most of the head space has been purged with CO2. Then I pull out the stopper and add an empty mylar balloon that is attached to a shot bit of stainless tubing that fits tightly in the grommeted hole. The balloon fills with CO2 with plenty of capacity to cover the suckback during cold crash.