Hi,
Just a Suggestion, You might try a cooler {styrofoam box w/lid}. Collect your dew and add the salt and place your bottle in the cooler. Now place some ice about 6 to 12 inches away {a solid block lasts longer than cubes}. Put the lid on and place the cooler in the coolest part of your house and out of the sun. Providing that you place a thermometer in the cooler it will be easier to keep track of the temp. Just replace the ice before the it melts completly {around 30 % left} it should allow you the time to complete the first stage of the process.
REMEMBER: All you have to do is keep the temp below 65F { 18 C } or below and you should be fine.
Remember to replace the ice!
Hope this Helps.