Answer: There are several strategies you can try. You will probably need to experiment and see what works best for you. Since the roots are not as well insulated as they would be in the ground, the danger is not just extreme cold but also and maybe more important, oscillating temperatures.
You need to pay attention to watering so they do not dry out. You should check the soil any time it is not frozen. You will water less during cooler weather,but you may still need to water. Wind can be drying, so wind protection is important, too.
The larger the container, the better the insulation, so larger containers are far better. Or, some gardeners will heap mulch around the containers taking care not to smother the crown of the plant.
If you can move them to a location in the shade such as on a north facing balcony, that is better. The reason for this is that once they get cold, they will be more likely to stay evenly cold in the shade since there is no solar heating during the day.
A small plastic green house is likely to overheat very fast during sunny days and then be too small to hold much heat at night, so I don't think that will help much.
Some gardeners have improvised insulating shelters using mulch or oak leaves or dry straw inside a burlap wrap and topped that with plastic to keep it dry. This insulates but also provides wind protection. You need to allow for some air exchange so it does not overheat (like a parked car on a sunny day) and to avoid condensation inside it. You should also check for mice, since this is a perfect home them. You would wait until very late fall, in NY probably Christmas, to put them in. They should be dormant and pretty much frozen. Take them out in early spring so they can wake up naturally with the season. If you enclose them too early/too late they are more likely to rot.
I hope this helps!
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