Hi Chris,
What a neat project! Here are a few of my thoughts:
1. Resin will not cast well into plastic bags. Assuming you could get it to release (I’m not sure there is enough mold release for that), I would expect the heat of the resin to melt the bags.
2. Another problem with casting into the bags is that will want to lie flat. They aren’t going to stay ‘puffed up’ like the pillows shown here.
If I were to attempt this project, I would find a few pillows I liked and mold them. You could use a brush on silicone to make the mold, then build a support box to hold the mold. (We don’t sell a brush on silicone, but you can try Smooth On. Last I knew, they had something that could do that.)
As for casting, I would use the Resin Obsession deep pour resin. https://shop.resinobsession.com/collections/resin/resin-obsession-deep-pour-resin It is meant for molds. It will cast clear (like shown) and if you are careful in mixing, should be bubble-free as well.