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Simple wax very useful from bee hive or Wal-Mart ;-)
Excellent points neondog!
I have used simple very cheap "tea candles" in a <$2 aluminum cook pot (from thrift store) under my tarp to reduce the condensation moisture and add a bit of warmth. Here is a photo I posted earlier (I think?) to prevent the candles from blowing out I stacked "dry" river stone around the candles which heated up and continued to radiate heat after the candles burned out. Homemade candles, multi-wick in a tuna or pet food can work better (may post a photo of that someday, probably not). This and other techniques have been used for hundreds of years NOT MY ORIGINAL IDEA OBVIOUSLY!
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As a teenager I also used about a 1/4" of wax to coat the tops of random glass jars of fruit jelly/jam when I ran out of mason jars with new lids (was overseas and had to import that stuff). This worked just fine. In the wilderness I have used disposable aluminum bottles like those sold with beer and soda.
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Because top is small, you need less wax. Can also heat up blade, knife, hatchet etc beside fire and seal a mylar bag with food in it. Obviously should sterilize container and food to destroy all bacteria and other pathogens before sealing the container. I have never tried this in a thick plastic water bottle rated at well over 200F but it might work. A cheap plastic water water bottle would be a "FAIL".
Necessity breeds creativity. If you live next to a shopping mall and have mommies' credit card you may be less creative, just sayn'.