Mulia,
I envy your determination in such an unpredictable place.I was wanting to know if you have mortar or clay bricks? You could stack these up inside your box used as an incubator,they too will absord heat when your electricity is working,and will hold the warmth longer than water bottles. Or perhaps several small rocks or stones you may find around your area. Filled water bottles will work,but I think water is a much better transfer of heat than even air is,,that is why almost all automobile engines uses water as a way to cool them. I would suggest making a circle of these bricks,with your eggs in the center of all them.Like a hens nest. The more bricks you can put insde of your incubator,the more heat they all will absorb,and should take a lot longer for them to come down in temperature than water.