imo, forearm curls arent going to help grip strength that much. grip strength comes from your fingers. right down to that last tip of your finger. forearm curls are more of a wrist movement.
youre better off using one of these
and not just squeezing it, but holding it. and not squeezing with your hands trying to make a fist, but like you're pulling in your fingers to your thumb.
i used to have one of those on the coffee table and id just play around with it while watching tv subconciously
i get my grip strength by wheeling wheelbarrows, handling tools, and fighting rocks all day. i lol when i can pinch grip a 3" thick 35lb allan block from in my truck box over the side one handed. while my employee has to get in the box and pick them up 2 handed.