Air pressure at airliner altitudes is still about 20-30% of the sea level value. That means 20-30% of the atmosphere is above that—a column of mass equal to 2-3 meters of liquid water.
A thin lead sheet would be a rounding error next to that.
This is an oversimplification that's rather wrong, but: a decrease in altitude of just 300 meters, at airliner levels, puts an additional atmospheric mass equal to ~1 cm of lead (Pb) above your head.
Have you seen the explanations of radiation where they say flying (as a passenger) is about equal to the dosage of a dental X-ray (or something similar)? Someone who spends their career getting exposed at that rate might be worth making them a shield.
It's *not possible* to make a shield. The cosmic radiation that reaches that altitude is highly energetic and highly penetrating—enough so to go through 3 meters of water—and would be completely unaffected by lead sheets. Any easily-shielded types of cosmic radiation have already been blocked by the atmosphere.
It's not worth it. Like, you could mandate planes fly 100 meters lower, but a lead coating would be so heavy. Let's say a 737 with a max weight of 70 tons. Covering the top half with lead would mean a 200 square meter sheet, and at 1cm thick it would weigh more than 20 tons.
A thin lead sheet would be a rounding error next to that.
This is an oversimplification that's rather wrong, but: a decrease in altitude of just 300 meters, at airliner levels, puts an additional atmospheric mass equal to ~1 cm of lead (Pb) above your head.