Does this imply that the all plastic gamer PCs (that are more designed to be art projects than serious computers) are bad and that everyone should be using systems with solid steel cases to block radiation?
The computers in plastic cases can sometimes be affected by appliances with electrical motors that are used nearby them.
The cosmic radiation is attenuated only inside a big building, e.g. one with many stories above you. A really great attenuation is obtained only underground.
But overall, pc cases are probably too thin to appreciably affect the radiation levels. Cosmic rays and their secondary radiation is fine with going through buildings so a 1mm thick case isn't going to do much.