[supernova87a]

Water is also a good protector. And the polyethylene is actually a proxy for "hydrogen atoms". The reason is that dense hydrogen has a lot of protons to interact with the incoming radiation.

But unfortunately plastic bags are not dense polyethylene. You would kind of need full blocks of solid polyethylene...

[teachtwolearn]

You can make solid polyethylene out of plastic bags by pressing them in a mold heated to ~100-150C.

But it's quite flammable, so you might not want to use it as a building material.