Re toxic fumes from burning plastic:

It depends on how complete the combustion is. At 100%, all that gets produced is carbon dioxide and water.

If you have a good hot fire going and the plastic has lots of surface area (like ziplock bags), combustion is essentially complete. If you have a smouldering fire, or the plastic is in a solid chunk, then there can be some harmful emissions.

FWIW it is against BWCA regulations to burn trash.

This past summer I used several 8oz, 16oz and liter sized light plastic bottles that you initially buy things like salad dressing, catsup, water, juice, or soda in.  I repackaged ground coffee, coffee creamer and sugar, cooking oil, pancake mix, pancake syrup, and so forth into them.  They are sturdy, generally do not leak, and you can pour out the contents conveniently.  I also use 1oz, 2oz and 4oz (Bailey's Minis) individual drink alcohol containers for very small amounts of some items like vinegar, olive oil, etc .