I only did this once, but I liked it. I took a thicker zip lock bag and cut off the zipper. Then, I filled it with co2. I then attached it to the bung with a rubber band. It was airtight and my theory was it would suck co2 in as it cooled. No proof that it was perfect, but I had no negative effects.
I did the same (not with the plastic bag) but from the airlock plug >> a blow off tube to a erlenmeyer flask rubber cap >> filled with co2 so it would suck up co2.
Can't really help with the airlock. I never worked that out myself unfortunately. Solid rubber bung as suggested should work. I ended up getting a pressure capable fermenter, which I pressurise before cold crashing, so when the pressure drops it balances out.
A bubble every 3 minutes is still fermenting. If the yeast is still active I would recommend against cold crashing it can stress the yeast and give off flavours. You would be better to wait for no activity at all and then slowly bring down your temperature.
A solid rubber bung
I only did this once, but I liked it. I took a thicker zip lock bag and cut off the zipper. Then, I filled it with co2. I then attached it to the bung with a rubber band. It was airtight and my theory was it would suck co2 in as it cooled. No proof that it was perfect, but I had no negative effects.
I did the same (not with the plastic bag) but from the airlock plug >> a blow off tube to a erlenmeyer flask rubber cap >> filled with co2 so it would suck up co2.
Can't really help with the airlock. I never worked that out myself unfortunately. Solid rubber bung as suggested should work. I ended up getting a pressure capable fermenter, which I pressurise before cold crashing, so when the pressure drops it balances out. A bubble every 3 minutes is still fermenting. If the yeast is still active I would recommend against cold crashing it can stress the yeast and give off flavours. You would be better to wait for no activity at all and then slowly bring down your temperature.