The pot I've been using recently is too big to fit in the sink, and winter has gone so there are no snow banks outside. I thought about using sanitized plastic bottles or glass bottles, but after concerns about sanitation with the first (removing labels leaves sticky goo that I imagine bacteria would love hiding in) and thermal shock failure with the latter, I decided to double bag the bottles. It then took me about 2 minutes to realize I should just ditch the bottles and use the gallon ziplocks.
My conclusion is that its honestly not really even worth freezing the water or putting ice in alongside the water. When you are cooling 3 gallons from 200 degree to 100 degrees. Its not going to make much of a difference whether you are immersing 6 quarts of a 30 degree substance or 6 quarts of a 40 degree substance. I probably used about 4 quarts of ice/water mix, before switching to cold water, I then put two bags with about 3 quarts of cold water eacher in, refilled once, and had hit 100 degrees. Mixing this with two more gallons of cold water in the fermenter got me right to 78 degrees.
The process took under 30 minutes, and I imagine it could be done in 15 or less if you had the bags of water prepared beforehand and switched them out as soon as they warmed up. The other advantage is that you can take the temperature of the warm water in the switched out bags rather than leaving the cover off the pot while you get a reading from the pot.
If I did it again I would use cold water first (if not only) and then maybe use a bag of ice towards the end as the temperature differential gets smaller. I was a little sketched out about achieving full sanitation with the weird opening of the bags, but I think I was probably being paranoid. Next time I might boil the bags first.