Realistically what am I supposed to do for my neck gains when I need to move up from a 20kg plate?

As part of my accessory day I do neck flexion and extension, and for both of those I've moved up to a 20kg plate, I'm getting around 20 solid reps doing both. What would you guys be doing when it's time to move up on weight? Am I supposed to try and hold a small plate on top of the big one? Or is there a better way? My goal is to add a bit more size to my neck, but not past a point where it's unsafe.