I've noticed that. I don't know if the the armor has changed since I bought it (purchased it from SC about four years ago), but I believe the instructions actually called for two rivets: one to hold the tank to the backplate (it didn't extend far enough for one rivet to go through both the greeblie and back plate) and one to hold the greeblie to the tank, and I ended up just using the one and gluing the topper. Would it be sufficient to attach a rivet going through the greeblie hole?
Unfortunately, it's a pretty strong plastic epoxy, I've had no luck cutting with a knife.