I don’t think this is a good idea. Plastic bags generally contain silicone in some form or fashion. Silicone is the kryptonite of epoxy. Silicone shouldn’t be within 100 yards of a rod builder - it is evil.
This would also apply to most souffle cups, stir sticks, straws, cup lids and other items gathered at food establishments. Most consumer plastics contain the stuff, you actually have to be cautious about what you use to mix epoxy.
FYI, wood products are unbelievably great at introducing air bubbles - the wood is porous and contains them. You might want to try using a stainless steel spatula for mixing and applying epoxy - it does a far superior job. Like anything else, it takes practice to learn to apply epoxy with a spatula - but once you do, you will never use a brush again - the finish will be immaculate.
I have a question for the epoxy experts out there:
I was always told not to use wood like Popsicle sticks, matches, toothpicks to mix epoxy because the epoxy will pick up stuff from the wood and eventually turn the epoxy an amber color.
I don’t build rods, (thank god, I have enough bad habits), but I use a lot of epoxy for tying and other stuff and stopped using toothpicks for this reason after some glue jobs turned amber.
Pick up a bunch of plastic swizzle sticks from your local watering hole. I use them and have never had a problem. Additionally, mixing with them tends not to create bubbles if you’re careful