As counterintuitive as this might sound, pleather and plastics (depending on the molding process) will need to be aired out to prevent degradation from accelerating. And these Batman figures are really not suited for long-term MISB storage.
What I did, was unbox my rubber figures, and I wrapped each in a few sheets of non-acidic paper. I also tossed in some silica packs. The wrapping keeps them dust free, but paper is porous enough to ensure that the material can air out. You could also try something like that. However, keeping these figures in pristine condition for years to come is just not going to happen. The best you can do is just slow down the degradation.
All of that said, I sometimes ponder about why I never transitioned fully into statue collecting...