I have supersaturated the brine with salt, so with me there is still salt on the bottom which has not dissolved, above is always a thick soft layer of ice, like snow slush and according to the thermometer -21-23°C cold. With acetone I could try even deeper, but that's enough for me. I also use the brine to cool my flask during exothermic reduction, I use the same brine all the time.
If you need to go even lower, find out about dry ice deliveries in your area, the dry ice can probably be used for up to 48h and also delivered on schedule.
Yes you can, if it is big enough, a dewar is the same only two times and more isolated in industry standard, I got one cheap at a second hand onlinestore in the correct size for my trap.