I used to do a wrist roller curl. use a wooden dowel, like a broom stick about 12" long. Attach a rope in the middle and make it long enough to just reach the floor while standing up with your arms at waist height. attach a weight plate, as heavy as you want. What you would do is start with winding up/down the weight on the rope while keeping your arms stationary, using only your wrist, hands, and forearms.