There's really only two ways to physically lower the likelihood of bit flips from cosmic rays, one, redundancy, and two, increasing the distance between transistors. One of the downsides of making transistors as small as they are nowadays, they are much more prone to bit flips. This is due to when a cosmic ray proton at high energy interacts with materials, you can get essentially a "splashing" effect of electrons around where the proton went through the material. By having transistors as close as possible this splashing or shockwave of electrons has a higher likelihood of electrons flowing into the transistors imparting a charge and causing a 0 to flip to a 1. Redundancy is important as a cosmic ray interacting with one computer chip wouldn't have any effects on another one nearby and like you said for error checking as well.

You could of course try to physically shield the computer, but trying to stop a cosmic ray proton is far easier said than done as they can travel at more than 50% the speed of light depending on energy. It can also be even worse if you don't stop the proton fully as cosmic rays stop both kinetically and electromagnetically causing the cosmic ray to impart more of its energy into the material than if it was at much higher energy. This is called the Bragg peak and is important in proton beam therapy for treating cancer.

Source: I worked in cosmic ray interactions with materials and semiconductors for my undergrad school's CubeSat program.