Side plate tips
Side plates add extra weight to the sides and can often cause the cumberbun to sag in the back. This is a common issue with ALL plate carriers. Here's a handy solution that might work for you (it works for us).
Get some paracord/550 cord or any cordage you may have laying around. After you get your cumberbun how you want it, rig a few loops around the MOLLE webbing and secure it there. You don't want to over-due it. It needs to sag a little so that the elastic can still stretch a little when you bend over or take a deep breath! This setup has worked great for us even when not running side plates. It also helps if you have a lot of gear on your cumberbun. We melt the ends of our paracord with a lighter.
This is the back of a Viper Modular Plate Carrier.