[ortusdux]

That linked nature paper seems to indicate 10g/cm^2 for a 50% reduction. A standard shopping bag is about 5g, so you would need roughly 20k bags/m^2, or ~2k bags/ft^2. At 0.9g/cm^3, that would be roughly 11cm of solid polyethylene

[robbmorganf]

I just squished a plastic bag as much as I could, and got it down to an inch cubed. So if I hypothetically did that for 2000 bags in 1ft^2 (sorry for English units), I think that would mean it could be 13-14in thick. So maybe it is reasonable to attenuate radiation by about 50% in a rather generous drop ceiling?

[tyingq]

Ah, so bags are out, as is practically the ceiling. But cut sheet HDPE is easy to find, so tiles atop your machine would be relatively cheap and easy.