Grip Extensors

I've started training more and more grip strength in my back workouts. I'm trying to work on false grip too. It seems that all this flexors training can cause some strain in the opposite side of my forearms, can anyone help me out with some hand/wrist/forearm extensors exercises (or tips) to balance out my training? All I can think of are those elastic things the internet markets and this video I watched of Brian Shaw who got a bucket of sand stuck his hand in with a closed fist and then opened his hand, doing so repeatedly.