בס"ד Can I boil an egg in a non kosher pot if I use two layers such as a zip lock bag?
ב"ה Shalom Plastic bags,are porous to a certain degree to the best of my knowledge and this would allow transference from the non- kosher pot. Also, when boiling something, I don't think you can be totally assured that the seal won't open or the bag perhaps will melt a bit. Therefore, the idea of using two layers does not solve the problem and is forbidden. The solution, of using a double seal, is applied when using dry heat such as heating food in a non-kosher oven which is the common solution when eating kosher hot food on a plane. (See Yoreh De'ah 108:1, Sefer Hakashrut Chapter 22:7) All the best and Shana Tova