If we make a square box with lead thick walls (1 meter thick or more), can me have a space inside this box 100% (or 95%) free of Cosmic microwave background radiation or it is something which we cannot isolate and it is property of the space and even in the box like mentioned above it will appear again somehow?
UPDATE 1
Clarification: I assume that I can distinguish between CMB and blackbody radiation of the box itself (maybe it is not possible, but it is theoretical questions anyway. Let's assume that I can :)). I can make box temperature high (500K, 1000K) or low (0.1K), that blackbody radiation of the box itself will not interfere with CMB.