Good suggestion from OldTimer1. That weight on one end of a DB has been used quite a bit, back in the day, by strongmen.
Can also do the same exercises with a short handled sledge hammer. And with a better grip. The longer the handle, the better for increased leverage. These dead blow hammers usually run for 2lbs to 10lbs for the longer handled version. Blacksmiths,heavy duty welders/repair men and forge workers (past & present) noted for the development and strength of their grip. Doing this type work all day long, your going to get that exceptional power. Ever done serious work with a sledge hammer, you will understand what I mean.
Can also use a dowel (thick handles preferred) with a rope attached to the end of the dowel and a BB plate on the other end of the rope. Just to note, lumber mill and yard workers will have good forearms and grip, from their daily hard work.
Just a personal view..no big deal: first choice for increased forearm size, for BB'ers, would be the wrist roller. Reverse BB curl (alternate regular and false grip) second. Developing forearm size does not always go with developing strength in the grip. Good Luck.